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VALVE GEARS 



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McGraw-Hill DookCompai^ 

Puj6Cts/iers qf3oo^/br 

Electrical World TheEngineei-in^ andMinin^ Journal 
EngiriGering Record Engineering News 

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Signal EnginoGr , American Engineer 

Electric Railway Journal Coal Age 

Metallurgical and Chemical Engineering P o we r 



VALVE GEARS 



BY 



CHARLES He^ESSENDEN, M. E. 

ASSISTANT PROFESSOR OF MECHANICAL ENGINEERING 
UNIVERSITY OF MICHIGAN 



First Edition 



McGRAW-HILL BOOK COMPANY, Inc. 
239 WEST 39TH STREET, NEW YORK 

6 BOUVERIE STREET, LONDON, E. C. 

1915 









Copyright, 1915, by the 
McGraw-Hill Book Company, Inc. 




FEB "8 1915 

THE. MAPLE . PRESS. YORK. PA 

©CI,A391610 



PREFACE 

It has been the mtention of the author in preparing this book, 
to present an elementary treatise on the subject of valve gears. 
Only steam engine valve gears are discussed, and, while the effort 
has been made to cover the best known and most representative 
types, the aim has been to produce a book suitable as a text 
rather than a reference book. The subject matter has been used 
in the classes at the University of Michigan for several years. 

The Bilgram and Zeuner diagrams are both used throughout 
the book and given almost equal consideration. Rather than 
introduce a third diagram, the Reuleaux, some of the most useful 
constructions belonging to that diagram have been incorporated 
in the Zeuner solutions. 

The author desires to express his thanks to his brother. Pro- 
fessor E. A. Fessenden, of the University of Missouri, and to his 
colleagues. Professors S. J. Zowski, W. F. Verner, and J. E. 
Emswiler, of the University of Michigan, for their assistance in 
preparing the work, and to the various manufacturers who have 
kindly furnished illustrations of their products. Professor E. R. 
Hedrick, of the University of Missouri, was of material assistance 
in reading the proofs, and made many helpful suggestions and 
criticisms. 

C. H. F. 

Ann Arbor, Mich., 
January, 1915. 



w* 



CONTENTS 

Page 
Preface v 

CHAPTER I 

Introduction 1 

Relative positions of the piston and crank — The valve gear of a 
simple, high speed engine — The action of the valve^Steam lap — - 
Exhaust lap — Port opening — Lead — Angle of advance — Relative 
positions of valve, eccentric and crank. 

CHAPTER II 

Valve Diagrams 17 

The Zeuner diagram — The Bilgram diagram. 

CHAPTER III 

Use of Valve Diagrams 25 

Five typical problems solved by both diagrams. 

CHAPTER IV 

Port Openings and Passage Areas 40 

Discussion of nominal steam speeds — Angular movement of con- 
necting rod — Overtravel. 

CHAPTER V 

Forms of Slide Valves 58 

Piston valves— Multiple-ported valves — Allen valve — Double- 
ported piston valve — Double-ported flat valve — Ball and Wood 
telescopic valve— Four-ported valve — Skinner engine valve — ^Loca- 
tion of ports in the valve seat — Sliders and rockers. 

CHAPTER VI 

Shaft Governors 73 

Westinghouse governor — Buckeye governor — Rites governor — 
Robb-Armstrong governor — The action of a governor shown on the 
valve diagrams. 

CHAPTER VII 

Valve Setting 80 

Setting by measurement — Setting by the indicator diagram. 

vii 



viii CONTENTS 

CHAPTER VIII 

Page 

The Design of Slide Valves 84 

Equal leads and unequal cut-offs — Unequal leads and equal cut-offs 
— The design of a D slide valve — The design of a flat double- 
ported valve — Investigation of the action of a shaft governor. 

CHAPTER IX 

Valves with Riding Cut-off 100 

The Meyer valve — The Buckeye engine valve gear — The Mclntosh- 
Seymour engine valve gear. 

CHAPTER X 

Pump Valves 114 

The Cameron pump — The Worthington duplex pump — The Blake- 
Knowles valve gear. 

CHAPTER XI 

Reversing Gears 120 

The Stephenson link motion — The Walschaert valve gear — The 
Joy valve gear. 

CHAPTER XII 

Corliss Valve Gears 136 

General arrangement — Forms of valves — The wrist plate — The 
valve motion — Trip gear — Use of two eccentrics — Design — The 
dashpot — Four-valve engine. 

CHAPTER XIII 

Poppet Valve Gears 159 

Poppet or lift valves — Poppet valve gears — Rolling levers — Rotat- 
ing cams — Oscillating cams — Layout. 

Index 169 



VALVE GEARS 

CHAPTER I 
INTRODUCTION 

Probably the part of a steam engine most interesting to the 
layman as well as to the experienced engineer, is the valve gear. 
This is particularly true of the more complicated arrangements 
such as are used on Corliss and other 4-valve engines, locomotives, 
reversing engines, poppet valve engines, blowing engines, etc. 

The action of the valve is of primary importance in the opera- 
tion of the engine since its function is to control the flow of steam 
into and out of the cylinder. In order for an engine to operate 
successfully and efficiently, a definite amount of steam, pro- 
portional to the load being carried, must enter and leave the 
engine cylinder at definite, predetermined times in the working 
cycle of the engine. The study of valve gears is essentially a 
study of the relative motions and simultaneous positions of the 
piston, crank and valve. 

Fig. 1 shows a single cylinder high speed engine. A piston 
is moved backward and forward by the steam and transmits 
reciprocating motion through the piston rod to the cross-head. 
The connecting rod communicates this motion from the cross- 
head pin, to the crank pin. The latter moves in a circle, called 
the crank circle, whose radius is equal to one-half the stroke of 
the piston. Thus the reciprocating motion of the piston is 
changed into rotary motion by the crank, and the shaft caused 
to revolve. 

Relative Positions of the Piston and Crank. — In Fig. 2 the 
full lines represent the moving parts of the crank and connecting 
rod mechanism at the beginning of a stroke. As the crank pin 
moves from A to A', the connecting rod moves from CA to C'A', 
the point C being found by striking an arc, with A' as a center 
and with a radius equal to the length of the connecting rod, 
cutting the center line of the engine. While the crank pin turns 

1 



VALVE GEARS 



from A to A' the cross-head pin and the piston move through the 
distance x. 




Conversely, if the distance through which the piston has 
moved is known, the corresponding position of the crank pin 



INTRODUCTION 



can be found by striking an arc, with the center at C and a radius 
CA, cutting the crank pin circle at A'. This arc also cuts the 
center line of the engine at m and it will be noticed that Am is 
equal to x since the connecting rod could be disconnected from 
the crank and the end A' rotated about C without any change in 
the position of the piston. 

In finding the relative positions of the piston and crank the 



C iC' _. 












FiG. 2, 



right hand end of Fig. 2 is sufficient. Suppose it is required to 
find the crank position when the piston has traveled 0.6 of its 
stroke. In Fig. 3 let A 5 represent the length of the stroke, then 
0.6 AS equals x. The circle on AS as a diameter represents the 
path of the crank pin. With a radius equal to the length of the 
connecting rod, and a center on the center line of the engine, draw 
an arc through m cutting the crank pin circle at A' . Then OA' 



-Length of Connecting ffod 




Fig. 3. 

is the crank position required. If the crank position is known and 
the corresponding piston position required, the process is reversed. 
It is immaterial what scale is used in representing the stroke, 
provided the connecting rod length is taken to the same scale. 

PROBLEM 1 

Plot a curve showing piston displacements as ordinates and crank positions 
as abscissae for each 30° of rotation of a 10" X 10" — 200 R.P.M. engine; 
the ratio between the lengths of the connecting rod and crank is 5 to 1. 



4 VALVE GEARS 

The Valve Gear of a Simple, High Speed Engine. — The 

entrance of steam to an engine cylinder and its exit from the 




Fig. 4. — Eccentric. 




Fig. 5. — Analogy between crank and eccentric. 

'Cylinder are regulated by one or more valves. Attention is 
called to the valve gear side of the engine illustrated in Fig. 1. 



INTRODUCTION 



The valve derives its motion from the crank shaft through the 
medium of an eccentric, eccentric rod, and valve stem. The entire 
mechanism used to operate the valve is often designated under 
the collective term valve gear. 

Fig. 4 shows the common form of eccentric; its movement is 
the same as that of a crank of 
length r, but it can be attached 
anywhere along a shaft while a 
crank can only be used at the end 
of a shaft. The ordinary form of 
crank is shown in the upper part 
of Fig. 5. If the pin, A, is greatly 
enlarged it will finally include the 
shaft as shown in the lower part of 
the figure, but the center of the pin 
can still revolve about the shaft 
as it did before. Thus it is seen 
that the eccentric is essentially a 
crank with the pin enlarged to in- 
clude the shaft. The inner portion which rotates with the shaft 
is called the eccentric sheave, and the bands which surround the 
sheave and in which it rotates are called the eccentric straps. 




Steam chest and D slide 
valve. 




D slide valve. 



The eccentric straps are fastened to the eccentric rod, whose mo- 
tion is similar to that of the connecting rod, and the rotary mo- 
tion which the eccentric receives from the shaft is transferred 
into reciprocating motion at the pin M (Fig. 1). The pin M is 



6 



VALVE GEARS 



fastened to a slider which guides it, and communicates its motion 
to the valve. 

Fig. 6 shows the steam chest of an engine with a part section 
of the valve. The type of valve shown is the D slide valve 
which is so called because its sectional outline resembles that 
letter. Fig. 7 shows the valve alone. 

PROBLEM 2 

Describe the construction of the valve gear on the engine. 

The Action of the Valve. — The action of steam in an engine 
cylinder may be studied from a diagram called an indicator 
card which is obtained from the engine during its operation and 
shows the steam pressure in the cylinder corresponding to any 
position of the piston. Fig. 8 shows a typical indicator card. 
The ordinates represent the steam pressures and the abscisssB 



Volume of 
Clearance Space -' 




Pressure (Absolute) 



Fig. 8. — Indicator card. 



the corresponding volumes. Besides representing the volume 
swept out by the piston, L also represents the stroke. Just 
before the beginning of the stroke, the valve opens the port leading 
into thQ cylinder and the steam pressure behind the piston rises 
to the maximum value B. The valve remains open and allows 
steam to flow in behind the piston, thus maintaining the high 
pressure shown by the approximately horizontal line BC, until C 
is reached. The point C is known as the 'point of cut-off. 
Here the valve closes, no more steam can enter the cylinder and 



INTRODUCTION 7 

the steam which is in the cyhndcr expands and the pressure falls 
as represented by the expansion curve CR, until the valve re- 
leases the steam and allows it to escape into the exhaust. The 
point R shows when this release occurs and is called the j^oint 
of release. The curve RE gives the pressures during the sudden 
expansion to the pressure in the exhaust pipe. 

During the return stroke, the steam, having performed its 
work and fallen to the pressure E, is pushed out into the exhaust 
pipe until the valve again closes the port leading from the 
cylinder. Any steam which is then imprisoned is compressed 
into the end of the cylinder and its pressure rises as shown along 
the curve KA. This curve is called the compression curve 
and K is the point of coinpression. When the piston is almost 
at the end of its stroke and while it is still compressing the 
steam left in the cylinder at compression into the clearance space, 
the valve admits a fresh supply, at A, and the operation is 
repeated. A is called the point of admission. 

A similar series of events occurs on the other side of the piston 
and the card for that end of the cylinder is shown dotted. While 
one side is being acted upon by live steam the other side is pushing 
out an exhaust charge. 

To draw the cards shown in Fig. 8 any scales may be chosen, 
but it is convenient to have the height of the card approximately 
half the length. Starting, then, with an initial pressure at B, the 
line from B to C should first be nearly horizontal and then slope 
down to represent the throttling or wire-drawing when the valve 
closes. From C to i^ the curve is drawn as a rectangular hyper- 
bola (with as an origin) because the actual diagram from an 
engine shows this curve to be very nearly a rectangular hyperbola, 
and it is a very simple curve to draw. To construct the curve, a 
radial line is drawn fromO, and horizontal and vertical lines from 
some point, such as C, through which the curve is to pass. The 
radial line cuts the horizontal and vertical lines and is used as the 
diagonal of a rectangle; the corner of the rectangle opposite C is 
another point on the curve. The compression curve from Kto A 
is also drawn as a rectangular hyperbola through the known point 
K. Fig. 8 shows cards for a non-condensing engine, since the back 
pressure is above atmospheric pressure. Due to friction in the 
exhaust pipes and passages, the pressure in the cylinder does not 
fall quite so low as at the discharge end of the exhaust pipe. 
Depending on the freedom of the exhaust this back pressure varies 



VALVE GEARS 




Exhaust 



H.E.Exh.Lap C.E.Exh.Lap 



H.LSt.Lap..^ \ 



■'■\x I'h'T- 



,-C.E StLap 



Mid Position 



-5- 
Mid Position. 




-2- 

H.E. Release 




-6- 
H.E. Admission. 



.Max. P.O. 



-3- 
Extreme Position. 



-7- 

Extreme Position. 




-4- 
H.E. Compression. 




H.E. Cut-off. 



Fig. 9. 



INTRODUCTION 9 

in practice from a fraction of a pound to about two pounds or 
even more in some cases. 

It is the function of the valve to time the events, admission, 
cut-off, release, and compression, properly, and to provide the 
necessary opening for the passage of the steam. The time of 
each event is expressed as a percentage of the stroke in which it 
occurs; for example, in Fig. 8 cut-off is said to occur at 25.8% 
of the stroke, release at 75%, and compression at 70%. 

The cylinder of an 8" X 10" - 250 R.P.M. engine, equipped 
with a D slide valve, is shown in Fig. 9 with the valve in its mid- 
position. By mid-position is meant the middle of its travel, 
half way between the two extreme positions. Below the cylinder 
drawing is a series of figures showing the valve in different im- 
portant successive positions for the head end of the cylinder. 
A similar series could be drawn for the crank end. In all valve 
gear work it is better to consider only one end at a time. 

It has become customary to reckon all valve movements from 
the mid-position. For this reason a drawing of a valve should 
show it in the mid-position. 

Steam must be let into and out of both ends of the cylinder of a 
double acting engine. When one valve does all this work it may 
be called a "four-function" valve; thus the D slide valve is a 
four-function valve, each of its edges having one function to 
perform. We may divide the work between two valves; each 
then becomes a two-function valve. A separate valve may be 
supplied for each function as is done, for example, on a Corliss 
engine. 

PROBLEM 3 

Draw a series of sketches showing the relative positions and direction of 
motion of the piston, valve, crank, and eccentric when: 

1. The valve is iQ. mid-position. 

2. The valve is in the admission position. 

3. The valve is in the extreme position. 

4. The valve is in the cut-off position. 

5. The valve is in the release position. 

6. The valve is in the compression position. 

Steam Lap. Exhaust Lap. — On the sketch showing the valve 
in mid-position (Fig. 9) , certain dimensions are marked steam lap 
and others exhaust lap. It is seen that before steam can be ad- 
mitted to the head end of the cylinder the valve must be drawn 



10 



VALVE GEARS 



to the right by the amount of the head end steam lap. Also 
before release can occur on the head end the valve must be moved 
to the left the amount of the head end exhaust lap. The valve is 
in the same position for admission as for cut-off, but its direction 
of motion is opposite for the two events. Likewise the position 
of the valve for release is the same as for compression, but the 
direction of motion is different. 

Steam lap is defined as the distance between the admission 
edge of the valve and the nearer edge of the steam port when the 
valve is in its mid-position. The steam lap is equal to the distance 
the valve is displaced from its mid-position when admission or 
cut-off occurs. 

Exhaust lap is defined as the distance between the exhaust edge 
of the valve and the nearer edge of the steam port when the valve 

is in its mid-position. The ex- 
haust lap is equal to the distance 
the valve is displaced from its 
mid-position when release or com- 
pression occurs. 

The steam laps on the two ends 
may or may not be equal and the 
same is true of the exhaust laps. 
The exhaust lap may be negative, 
in which case the valve fails to 
cover the port when in its mid-position. The width of port 
standing open is the negative exhaust lap. 

Port Opening. — In Fig. 10 the valve has been moved the dis- 
tance m from the mid-position, shown in dotted lines, to the 
position shown in full lines. The net port opening is the valve 
displacement minus the steam lap, or m — s = p. The port 
opening, p, is measured in inches. 

Lead. — ^Lead is the width of port opening when the crank is 
on dead center. 

The correct amount of lead depends on the size of the engine, 
its speed and the amount of compression. If the speed is high, 
considerable compression is needed to overcome the inertia of 
the piston and the other reciprocating parts, at the end of the 
stroke. The lead opening may admit enough steam to materially 
assist the compression in cushioning the piston. Another pur- 
pose of lead is to insure full steam pressure on the piston from 
the very beginning of the stroke. As some time is required for 




[<-— s— >l<-p>| 

H m — -H, 

Fig. 10. 



INTRODUCTION 



11 



the steam to fill the clearance space, admission must occur be- 
fore the crank reaches the dead center position. 

It is more logical to say that the admission should occur when 
the crank is from 8° to 2° before the dead center position, and 
to choose larger angles for fast running engines and smaller 
angles for slow running engines than to attempt to give definite 
values for the lead. The best criterion is the indicator card. 
If the lead is correct, the admission line will be practically vertical 
as shown in Fig. 11a, because admission occurs so shortly before 
the dead center position is reached that the distance of the 
piston from the end of the stroke, when reduced to the scale 




Fig. 11. — Admission lines. 

of the indicator card, is scarcely more than the thickness of 
a pencil line. 

If the admission line is not vertical but leans to the left as in 
Fig. lib, it shows that the valve opens too soon. Less lead will 
make the line more nearly vertical. Fig. lie shows deficient 
lead but admission occurring before dead center; the steam is 
tardy in reaching its highest pressure behind the moving piston. 
Admission lines as shown in Fig. lid are sometimes obtained due 
to the condensation of some of the steam which was compressed 
into the clearance space, or to its leakage past the piston, fol- 
lowed by a later admission than shown in Fig. lie. If neither con- 
densation nor leakage past the piston occurs, but the admission is 



12 VALVE GEARS 

after dead center, the card will be similar to Fig. lie. Sometimes 
condensation shows on a card having a vertical admission line, 
by a small hook at the end of the compression curve (see Fig. 1 If) . 
This can be corrected by slightly increasing the lead. 

In horizontal engines the lead should be about the same on the 
two ends of the cylinder. In vertical engines the lead on the 
top, or head end, is usually considerably less than on the bottom 
end because the weight of the reciprocating parts acts against 
the steam pressure on the up-stroke. 

PROBLEM 4 

Measure the leads on the engine in the laboratory. Take indicator 

cards and write a report telling whether or not the leads are correct, and if 
not, how they should be corrected. 

The eccentric is shown in Fig. 12 as a crank of length r. The 
eccentric radius is always small compared with the length of the 
eccentric rod, so that the angle </> is always small. If, then, we 
say that when the end of the eccentric rod is at c', half way be- 



^^^^ ^^^^^^^f^^ ab = a' b'='7ota{ Movement of the 

■ '^wy/y/^^'-.. £/(i^aij5t ValvejOrltsJravef. 

Fig. 12. 

tween the two extreme positions a' and h' , the eccentric center 
is at c, 90° from a, the error caused by neglecting the extremely 
small distance OX will be very slight. The same assumption 
cannot be made for the crank and connecting rod, because the 
ratio of the length of the crank to that of the connecting rod 
is so large. Hence the angle ^ becomes relatively large and 
considerable error is involved in neglecting its versed sine. 

To admit steam to the head end the valve must be drawn to 
the right an amount equal to the head end steam lap. Move 
the eccentric center from c to s (Fig. 13), where os' is equal to 
the head end steam lap, and the valve will be in the admission 
position. 

Suppose the valve has a lead of L inches. Then it must stand 
open L inches when the crank is on center. Move the eccentric 
center to i, s't' being equal to L. Fig. 13 shows the relative 
positions of the valve, crank, and eccentric. 



INTRODUCTION 



13 



Angle of Advance. — The angle cot is called the angle of ad- 
vance, and is equal to the lap angle, cos, plus the lead angle, 
soL The angle of advance may be defined as the angle 
through which the eccentric must be moved to draw the valve 
from its mid-position to the position it should have when the 



Steam lap Angle. ^ 



fitngle of Advance 



^'•■^-Lead Angle 




Exhaust 
Cavity 



Fig. 13. — Relative positions of valve, crank, and eccentric. 

crank is on dead center. Sometimes the angle of advance is 
called angular advance. It is usually designated by 5. 



PROBLEM 5 

If the eccentricity is 2", the steam lap 1" and the lead iV"j what should 
be the angle of advance? 



14 



VALVE GEARS 



Relative Positions of Valve, Eccentric and Crank. — When 
the valve is in mid-position the eccentric is perpendicular to 
the line of stroke of the valve, as at om in Fig. 14. The motion 
of the valve is the projection of the motion of w on p'p. 

When a valve having steam lap and lead is ready to admit 
steam to the head end of the cylinder, the crank should be at a 
position OA (Fig. 14) shortly before the head end dead center. 



.Path of 
Eccentric Center 




J 
Fig. 16. — Release and compression. 

The center of the eccentric is then at a, 90° + 5 ahead of OA. 
This position of the eccentric is necessary to draw the valve 
from the mid-position, which is shown dotted (Fig. 15), into the 
correct position for admission, which is shown in full lines. As the 
eccentric center rotates from a to j), in Fig. 14, the valve continues 
to move to the right. When y is reached the valve is in its ex- 
treme position and the port x in Fig. 15 is widest open. The 



INTRODUCTION 15 

valve then starts to the left, and when the eccentric center 
reaches c the valve is again in the position shown in Fig. 15. 
This is the position for cut-off; c is directly below a, both points 
being a distance s (equal to the head end steam lap) from mid- 
position. 

As the eccentric center turns from c through m' to r, the valve 
moves to the left from the position shown in Fig. 15, through 
the mid-position, which corresponds to the eccentric position m' , 
to the position shown in Fig. 16. This is the position for head 
end release: the valve is to the left of mid-position the amount 
of the exhaust lap e. The valve continues to move to the left 
until the eccentric passes through the extreme position oy' ; 

/\ ^> — 



Fig. 17. 

then it starts to the right. At the eccentric position ok, the 
valve is again in the position shown in Fig. 16. This is the 
position for head end compression. 

The crank position for any eccentric position can be found 
by stepping back an angle of 90° rf- 5 from the position of the 
eccentric radius. To obtain the crank positions for all of the 
eccentric positions shown in Fig. 14 it is therefore only nec- 
essary to turn the entire figure backward through the angle 
90° -f 5. This gives Fig. 17, where the capital letters designate 
the crank positions corresponding to the eccentric positions in- 
dicated by the small letters in Fig. 14. The lines connecting a 
and c, r and k in Fig. 14 are parallel, and perpendicular to p'p, 
and of course the same is true in Fig. 17. If the valve travel and 
the crank positions for any three events are known, such as 
admission, cut-off and compression, draw OA, OC, OK (Fig. 17) 



16 VALVE GEARS 

connect AC, thus determining the head end steam lap s. Then 
draw a parallel to AC through K, thus locating OR, the crank 
position for release, and determining the exhaust lap e. 

PROBLEM 6 

What is the purpose of steam lap? 

What is the purpose of exhaust lap? 

In what way is the action of a valve with + 1/8" exhaust lap different 
from that of a valve with — 1/8" exhaust lap? 

How would negative exhaust laps on both ends of a valve affect the 
operation of the engine? 



CHAPTER II 
VALVE DIAGRAMS 

The operation of existing valves and the design of new ones 
can best be studied by the aid of accurate graphical construc- 
tions, called valve diagrams, which show the displacement of 
the valve from its mid-position at any time during the revolu- 
tion of the engine. Many diagrams have been devised, but the 
most common and convenient ones are the Zeuner and the 
Bilgram, named after their inventors. Each of these diagrams 
is popular, the Zeuner because it is older and gives more of a 




Fig. 18. Fig. 19. 

Figs. 18 and 19. — Fundamental idea of the Zeuner diagram. 

picture of the action of the valve, and the Bilgram because it 
is simpler for some design problems. Both are in such common 
use that it behooves all engineers to be familiar with both, if 
only to be able to read articles using either diagram. One who 
has much use for valve diagrams will eventually fall into the 
habit of choosing the diagram to suit his problem, and will quite 
often combine parts of both diagrams. 

The Zeuner Diagram. — Draw a pair of axes through (Fig. 
18). With as a center and a radius equal to the eccentricity, 
r, draw a circle. 

2 17 



18 



VALVE GEARS 



. Let Of be the eccentric position corresponding to any crank 
position OF. Then the displacement of the valve from mid- 
position is the distance qf. 

On the line Op as a diameter draw a circle. Then Ov is equal 
to qf because the triangles Oqf and Ovp are equal. 

By revolving all of Fig. 18 except the crank position OF 
counterclockwise through the angle 90° + 8, Of will be made 
to coincide with OF and Fig. 19 result. 

Then for crank position OF the valve is displaced from its 
mid-position the distance Ov. This is the fundamental idea 
of the Zeuner diagram; other points can be developed with this 
as a basis and will be readily understood provided this principle 
is firmly grasped. 




Fig. 20. 



Fig. 21. — Complete Zeuner diagram. 



Since the action of a valve in regulating the steam distribution 
is dependent upon the steam and exhaust laps, as well as upon 
the eccentric radius and the angle of advance, it is desirable to 
represent them on the diagram. For any crank position, the 
displacement of the valve from mid-position is the distance from 
to the intersection of the crank line, OF, with the circle of 
diameter r, sometimes called the valve circle. The net port 
opening is the valve displacement minus the lap. With as a 
center and radius equal to the head end steam lap, strike an arc 
as shown in Fig. 20. For any crank position such as OF the valve 
is Od from mid-position and the port opening is db. 

When the crank is at OA and turning in the direction of the 
arrow the port is just about to open, so OA must be the crank 



VALVE DIAGRAMS 19 

position for head end admission. When the crank has moved to 
the head end dead center position, OD, the port will be open the 
distance L, which is the head end lead. 

The port opening increases for a time as the crank revolves; it 
reaches a maximum when the crank is at P, and then diminishes 
and becomes zero when the crank reaches OC, the position for 
head end cut-off. 

Extend the line OP into the third quadrant and draw another 
circle of diameter r passing through 0. With as a center and 
radius equal to the head end exhaust lap strike an arc in the third 
quadrant if the exhaust lap is positive, or a dotted arc in the first 
quadrant if the exhaust lap is negative. 

When the crank reaches OR the displacement of the valve 
from its mid-position is just equal to the head end exhaust 
lap so this must be the crank position for the head end 
release. The exhaust port opening increases for a time, reaches 
a maximum, diminishes, and finally becomes zero when the 
crank reaches the position OK, the position for the head end 
compression. 

Thus far only the events for the head end of the cylinder have 
been considered. The diagram for the crank end is drawn simi- 
larly and shows the crank end lap circles and the crank positions 
for the crank end events, A complete diagram for both ends of 
the cylinder is shown in Fig. 21, in which the heavy lines and 
umprimed letters refer to the head end, and the light lines and 
primed letters to the crank end. The head end part of the dia- 
gram is reproduced from Fig. 20. 

The important points to be remembered in this construction 
are : • 

1. The angle of advance, 8, is laid off from the axis which is 
perpendicular to the travel of the valve, in the opposite direction 
from that in which the crank turns. 

2. The length from to the intersection of the line representing 
any crank position (or that line produced) and the circle of di- 
ameter r shows the displacement of the valve from the mid- 
position. 

In studying the action of a valve in connection with Figs. 14, 
15, 16 and 17, the Zeuner diagram was partly developed but not 
named. If circles are drawn on OP and OP' in Fig. 17, it becomes 
a complete Zeuner diagram. 



20 



VALVE GEARS 



Useful Characteristics of the Zeuner Diagram. — 1. A per- 
pendicular dropped from P (Fig. 20) will always intersect the 
horizontal axis through at a distance from equal to the 
steam lap plus the lead. This is evident from Fig. 22. The 
shaded triangle is inscribed in the circle, and one side is a 




Fig. 22. 



Fig. 23. 




24. Fig. 25. 

Figs. 22 to 25. — In these figures the circle about must have for its 
radius the eccentricity r. 

diameter; therefore it is a right triangle, and as previously- 
shown OX = steam lap plus lead. 

2. A perpendicular from P to the crank position for admission 
or cut-off intersects the valve circle at the same point as the 
steam lap arc (see Fig. 23) . As an example of the value of this, 
suppose the angle of advance, the eccentricity and the crank 
position for cut-off are known and that it is required to find the 



VALVE DIAGRAMS 21 

steam lap. Locate P, drop a perpendicular from it to OC and 
find the point Y ; then OY = steam lap. This construction is 
more accurate than that which uses the intersection of OC with 
the valve circle. 

Of course the same is true of the exhaust lap and the crank 
positions for the exhaust events. 

3. If OA and OC (Fig. 24) are the crank positions for admission 
and cut-off, respectively, a circle with Z) as a center and a radius 
equal to the head end lead is tangent to AC. ACis perpendicular 
to OP. OU = OX = steam lap plus lead. These relations will 
be readily seen by rotating Fig. 24 clockwise through the angle 
90° + 5 so that the crank positions shown become corresponding 
eccentric positions and Fig. 25 results. Fig. 25 is similar to Fig. 
13, which has been explained. 

The Bilgram Diagram. — Draw a pair of axes as shown in Fig. 
26. Call the eccentric radius r. When the crank is at OD the 
eccentric is at Od and the valve is the distance hd from its mid- 
position. 

Suppose the crank moves through any angle, a, to the position 
OF; the corresponding eccentric position is Of and the valve is the 
distance b'f from its mid-position. 

Lay off the angle of advance, 8, from the axis OD' in the 
opposite direction from that in which the crank rotates, thus 
locating the fixed point Q. 

From the fixed point Q drop a perpendicular, Qq, to OF (or OF 
extended). This perpendicular distance, Qq, is the displacement 
of the valve from its mid-position when the crank is at OF, because 
the triangle QOq is equal to the triangle b'fO and Qq equals h'f. 
This is the fundamental idea of the Bilgram diagram. 

Steam lap is represented on the Bilgram diagram by a circle 
with Q as a center and radius equal to the steam lap s (see 
Fig. 27). 

Exhaust lap is shown by a circle, about the same point Q as a 
center, with a radius equal to the exhaust lap e. If the exhaust 
lap is negative, the circle is shown dotted. 

When the crank is at OA (its extension tangent to the head end 
steam lap circle of radius s), the displacement of the valve from 
mid-position is the length of the perpendicular from Q to OA, 
which is s. The valve has been drawn to the right of mid-position 
by the amount of the head end steam lap, so that it is just ready to 



22 



VALVE GEARS 



admit steam to the head end of the cylinder. The crank posi- 
tion OA is the crank position for head end admission. 

For crank position OD the valve has moved the distance QB 
from mid-position. A portion QB' = s of this movement was 










Fig, 26. 



Fig. 27. 



Figs. 26 and 27. — Fundamental idea of the Bilgram diagram. 



required to draw the valve over the amount of the steam lap, so 
that the opening of the port leading to the cylinder is B'B. From 
the definition of lead, B'B must be the head end lead. 

For any other crank position, such as OF, the valve has been 

moved the distance QG from 
mid-position, and the port open- 
ing is GG'. 

For crank position OP (per- 
pendicular to QO) the displace- 
ment of the valve from the mid- 
position is a maximum and 
equals QO, or r. When the crank 
reaches OP, the valve is in its right 
hand extreme position; it then 
starts back to the left. When 
the crank is at OP, the eccentric 
is on the horizontal axis line. 
When the crank reaches the position OC, the valve is at the same 
distance s from micl-position as when the crank was at OA, 
but it is now moving in the opposite direction; OC is therefore 
the crank position for head end cut-off. 




VALVE DIAGRAMS 23 

When the crank reaches the position OQ, the valve is in mid- 
position. 

As the crank moves from OQ to OR, the valve moves to the 
left; when the crank reaches the position OR, the valve is to 
the left of mid-position by the amomit of the head end exhaust 
lap e, and is just ready to release the steam from the head end 
of the cylinder. Hence OR is the crank position for head end 
release. 

When the crank reaches the position 0P\ the valve is at the left- 
hand extreme position and its direction of motion changes. 

Compression occurs when the crank reaches the position OK, 
because then the valve stands to the left of its mid-position by 
the amount of the head end exhaust lap. 

The greatest port opening to live steam is evidently OM. 

This completes the discussion and analysis of the head end; a 
similar diagram can be drawn for the crank end, and it is given in 
Fig. 28 with the same notation primed. 

PROBLEM 7 

Given the eccentric radius 2^", the angle of advance 40° and the 
ratio of the length of the connecting rod to that of the crank 5 to 1. Using 
the Bilgram diagram for the outstroke and the Zeuner for the instroke, 
plot points showing the displacement of the valve from its mid-position for 
each 30° of crank rotation and connect the points by a smooth curve. Plot 
piston positions as abscissae and valve displacements as ordinates, measuring 
up from the base line for positions to the right of mid-position and down 
from the base line for positions to the left so that the curve will form an 
approximate ellipse. 

PROBLEM 8 

Given the valve travel 3", the head end steam lap 1", the head end lead 
1/16", the head end exhaust lap 1/16", and the ratio of the length of the con- 
necting rod to the crank 5 to 1. 

Determine the percentages of the stroke at which head end release and 
compression occur. 

In solving this problem use both diagrams drawn about the same center 
so that one solution checks the other. 

In solving valve problems by the Bilgram diagram it is often 
necessary to draw a circle tangent to two lines, OA and OC, 
and an arc, s, as represented in Fig. 29. This can be done by 
the "cut and try" method; but a better way is to locate the 
center by a simple geometric construction: 



24 



VALVE GEARS 



1. Bisect the angle between the lines OA and OC by the line 
OX. 

2. Erect a perpendicular to OX at the point E, where it 
intersects the arc s. 

3. Lay off DB equal to DE. 




Fig. 29. 



4. Draw BE perpendicular to OC, thus locating E the required 
center of the circle. E could be located also by bisecting the 
angle EBB. 

In connection with the Zeuner diagram it is sometimes neces- 
sary to draw a circle tangent to two lines, as Z)TF and WC, 

and passing through a known 
point P as illustrated in Figs. 
30 and 40. 

1. Draw WX bisecting the 
angle BWC, and with any center 
B, on this line, draw a circle 
tangent to the lines DW and 
WC but not passing through P. 

2. Draw a line from W through 
P, thus locating A. 

• 3. DrawAB. 

4. Draw PO parallel to AB, 
thus locating the required center 
0. 

It will also be noted that the 
triangles AEB and PEO are similar. This fact can be employed 
occasionally to obtain more accurate results than can be ob- 
tained by merely drawing AB and PO. 




Fig. 30. 



CHAPTER III 
USE OF VALVE DIAGRAMS 

The things shown on a valve diagram are: 

1. Crank position for admission OA 

2. Crank position for cut-off OC 

3. Crank position for release OR 

4. Crank position for compression OK 

5. Angle of advance 5 

6. Eccentricity r 

7. Valve travel 2r 

8. Steam lap s 

9. Exhaust lap e 

10. Port opening G 

11. Lead L 

In any problem certain items are known and the others are found 
by constructing the diagram. Almost any case will fall under 
one of the following five examples. Each problem has been 
solved twice to illustrate both diagrams. The notation given 
above has been used throughout; the crank end notation is 
distinguished from the head end by the prime (') mark. 

PROBLEM A 

Given. — Valve travel = 2r inches. 
Crank positions for 

admission, both ends, OA and OA'; 

cut-off, head end, OC) 

release, both ends, OR and OR' . 

Bilgram Solution (Fig. 31) for Problem A.^ — Draw a pair of 
axes intersecting at and with as a center draw the circle of 
radius r to represent the path of the eccentric center.^ 

^ To reduce inaccuracies and eliminate mistakes due to errors in scaling, 
valve diagrams should be constructed full size or larger. 

^ As only the crank angle is important in this work, it is immaterial what 
scale is used in representing the path of the crank pin. It simplifies the 
diagram somewhat if the circle representing the path of the eccentric is 
used to represent also the path of the crank pin to some reduced scale. 

25 



26 



VALVE GEARS 




'-■-i. 



Fig. 31. — Solution by Bilgram diagram. 



{ Valve Travel =B r. 
Given ; < ( Admission A eirtetA ' 

yCrank Position for \ Cut-off C. 

Release ffandR'. 




Fig. 32. — Solution by Zeuner diagram. 
Problem A. 



USE OF VALVE DIAGRAMS 27 

Draw the given crank positions OA (head end admission), 
OA' (crank end achnission), OC (head end cut-off), OR (head end 
release) and OR' (crank end release). 

The head end steam lap, s, is determined by the radius of the 
circle drawn with its center on the eccentric circle and tangent 
to the crank position for head end admission (extended), OA, 
and that for head end cut-off, OC. The center of the lap circle 
is the point Q from which the valve movements are measured. 

Having located Q, the angle of advance (8) is known. 

The head end exhaust lap, e, is the radius of the circle drawn 
with Q as a center and tangent to the crank position for head 
end release, OR. 

Draw the crank position for head end compression, OK, tangent 
(extended) to the head end exhaust lap circle but on the opposite 
side from OR. 

An arc with center at and tangent to the head end steam 
lap circle has a radius equal to the maximum port opening, G. 

The distance between the horizontal axis and a horizontal 
line tangent to the lower side of the head end steam lap circle 
is the head end lead, L. 

The center of the crank end lap circles is at Q', on the circle 
of radius r and diametrically opposite Q. 

The crank end steatn lap, s', is determined by the radius of the 
lap circle drawn with Q' as a center and tangent to OA' (extended), 
the crank position for admission on the crank end. 

Draw OC, the crank position for crank end cut-off, tangent to 
the circle s'. 

Draw the crank end exhaust lap circle, e', about Q' and tangent 
to the crank position for crank end release, OR'. 

The extended crank position for compression, OK', is tangent 
to the circle e'. 

The maximum port opening on the crank end is the radius G'. 

The distance between the horizontal axis and the horizontal 
line tangent to the upper side of the steam lap circle, s', is the 
crank end lead, L'. 

Having located the crank positions for the four events on 
both ends of the cylinder, the corresponding piston positions 
can be determined and the indicator cards drawn as shown. 

Zeuner Solution (Fig. 32) for Problem A. — Draw a pair of axes 
intersecting at 0, and with as a center draw the circle of radius 



28 VALVE GEARS 

r to represent the eccentric circle to one scale and the crank pin 
circle to some other scale. 

Draw the given crank positions for head end admission, OA, 
crank end admission, OA', head end cut-off, OC, head end re- 
lease, OR, and crank end release, OR'. 

Draw the line PP' bisecting the angle AOC. This determines 
the angle of advance, 8. 

Connect the points A and C on the eccentric circle by the dot- 
ted line AC; this will also be perpendicular to the line PP'. 

The head end steam lap, s, is found by striking an arc with as 
a center and tangent to the line AC. 

Drop a perpendicular from P to the horizontal axis. The 
distance between this line and the steam lap arc, measured on the 
horizontal axis, is the head end lead, L. 

Draw RK parallel to AC and perpendicular to POP' and then 
draw the crank position for head end compression, OK. 

The head end exhaust lap, e, is found by striking an arc with as 
a center and tangent to the line RK. 

The maximum port opening on the head end is the distance G. 

Similarly, for the crank end draw A'C and R'K' parallel to AC 
and perpendicular to POP'. 

Draw the crank positions for crank end cut-off, OC, and crank 
end compression, OK'. 

Strike an arc with center at and tangent to A'C Its radius 
is the crank end steam lap, s'. 

Strike an arc with center at and tangent to R'K', Its radius 
is the crank end exhaust lap, e'. 

A perpendicular from P' to the horizontal axis determines the 
crank end lead, L'. 

The maximum port opening on the crank end is the distance G'. 

PROBLEM B 

Given. — Valve travel = 2r inches 

Steam lap, head end = s inches 

Leads, both ends = L and L' inches 

Crank positions for release, both ends, OR and OR'. 

Bilgram Solution (Fig. 33) for Problem B. — ^Draw a pair of 
axes intersecting at 0, and with as a center and radius r draw the 
circle representing the path of the eccentric. 

Draw a line parallel to the horizontal axis and at a distance L 



USE OF VALVE DIAGRAMS 29 

(the head end lead) from it. Draw the crank end lead Hne in the 
same manner. 

Draw a hne parallel to the head end lead line and at a distance 
s (the head end steam lap) above it. The intersection with the 
eccentric circle locates the center, Q, of the head end lap circles 
and determines the angle of advance, 8. 

With center at Q draw the head end steam lap circle with 
the radius s. Tangent to this circle draw the crank positions 
for head end admission, OA (extended), and head end cut-off, OC. 

Draw the given crank positions OR (head end release) and OR' 
(crank end release). 

Locate Q', the center of the crank end lap circles, on the 
circle of radius r and diametrically opposite Q. 

Draw the head end exhaust lap circle, of radius e, with Q as a 
center and tangent to the crank position for head end release, OR. 

In a similar manner draw the crank end exhaust lap circle, 
of radius e', about Q' and tangent to the crank position for release 
on that end {OR'). 

The crank end. steam lap, s', is determined by the radius of the 
crank end lap circle drawn with Q' as a center and tangent to the 
crank end lead line. 

The crank positions for crank end cut-off, OC, and crank end 
admission, OA' (extended) are tangent to the circle s'. 

The crank positions for compression OK and OK' are tangent 
(when extended) to the exhaust lap circles e and e'. 

The maximum port openings on the two ends are determined 
by the radii of the arcs G and G' drawn tangent to the steam lap 
circles. 

Zeuner Solution (Fig. 34) for Problem B. — Draw a pair of axes 
intersecting at and with this point as a center and r as a radius 
draw the eccentric circle. 

Strike an arc with center at and radius equal to the head end 
steam lap, s. 

Lay off the head end lead L, on the horizontal axis from the 
point where the steam lap arc intersects it, and erect a perpen- 
dicular to locate P. 

Draw POP'. The angle of advance, 8, is now known. 

Draw the given crank positions OR (head end release) and 
OR' (crank end release). 

Draw RK perpendicular to PP', to locate K and then draw 
OK, the crank position for head end compression. 



30 



VALVE GEARS 




Fig. 33. — Solution by Bilgram diagram. 



G/i/en : 

Valve Travel ■^2r. 
■Steam Lap t= vS. 
Leads = Land L. 

Any Point in tlie Exhaust Diagmm, say 
Crank Positions for Release, R ancl/f. 




Fig. 34. — Solution by Zeuner diagram. 
Problem B 



USE OF VALVE DIAGRAMS 31 

Draw AC tangent to the steam lap arc, s, and perpendicular 
to PP' thus determining the crank positions for head end ad- 
mission, OA, and head end cut-off, OC. 

The head end exhaust lap, e, is determined by the radius of the 
arc drawn with as a center and tangent to the line RK. 

Erect a perpendicular from P' to the horizontal axis and from 
the point of intersection lay off, to the left, the crank end lead 
L'. Through the point so found draw the crank end steam lap 
arc, s'. 

Draw A' C tangent to s' and perpendicular to PP', thus deter- 
mining the crank positions for admission, OA', and cut-off, OC , on 
the crank end. 

A line through R' perpendicular to PP' locates K' and gives 
the crank position for compression, OK' . 

The crank end exhaust lap circle, e' , is tangent to the line 
R'K'. 

Finally draw the circles with PO and P'O as diameters to show 
the port opening at any crank position. 

PROBLEM C 

Given. — Valve travel = 2r inches 

Crank position for cut-off, head end, OC 
Crank position for cut-off, crank end, OC 
Lead, head end, L inches 
Exhaust laps, both ends, e and e' inches. 

Bilgram Solution (Fig. 35) for Problem C. — Draw a pair of 
axes intersecting at and the circle whose diameter is the valve 
travel, 2r. 

Draw the given crank positions OC (head end cut-off) and OC 
(crank end cut-off). 

Draw the head end lead line parallel to the horizontal axis and 
at a distance L (head end lead) from it. 

Draw the head end steam lap circle of radius s, tangent to the 
crank position OC, and the lead line. The center, Q, of this 
circle will be on the bisector of the angle between OC and the lead 
line and at its intersection with the eccentric circle. Locating Q 
determines the angle of. advance, 8. 

Draw the crank position for head end admission, OA, tangent 
(extended) to the steam lap circle, s. 

Draw the head end exhaust lap circle, e, with Q as a center. 



32 



VALVE GEARS 




Fig. 35. — Solution by Bilgram diagram. 



Given ' 

Valve Travel = 2r. 

Cat- off CandC! 

Lead L . 

Exhaust Lap e and e '. 




Fig. 36. — Solution by Zeuner diagram 
Problem C. 



VSE OF VALVE DIAGRAMS 33 

The crank positions for head end release, OR, and compression, 
OK, are tangent to the exhaust lap circle, e. 

Strike an arc with as a center and tangent to the head end 
steam lap circle. Its radius is equal to the maximum port 
opening, G. 

The center of the crank end lap circle is at Q\ on the circle of 
radius r and diametrically opposite Q. 

The crank end steam lap, s', is determined by the radius of the 
lap circle drawn with Q' as a center and tangent to the crank 
position for cut-off, OC. 

Draw the crank end lead line parallel to the horizontal axis and 
tangent to the crank end steam lap circle. Its distance, U, 
from the horizontal axis is the crank end lead. 

The crank positions for crank end release OR', and compression 
OK' are tangent to the exhaust lap circle e'. 

On the crank end the maximum port opening is G'. 

Zeuner Solution (Fig. 36) for Problem C. — Draw a pair of axes 
intersecting at and with as a center and the length of the 
eccentric arm, r, as a radius, draw the circle to represent the 
path of the eccentric center. 

Draw the given crank positions OC (head end cut-off) and 
OC (crank end cut-off). 

With as a center and with radii e and e' (the exhaust laps) 
strike arcs as shown in the figure. 

Draw a circle with Z> as a center and with a radius equal to the 
head end lead, L. Tangent to this circle and passing through C 
draw the line ^ C as shown. The intersection of this line with the 
eccentric circle determines the point A and the crank position for 
head end admission, OA. 

Bisect the angle AOC, thus locating the line PP' and determin- 
ing the angle of advance, 8. 

Draw the lines RK and R'K' perpendicular to PP' and tangent 
to the exhaust lap arcs, e and e', and in this way locate the crank 
positions for release (OR and OR') and compression (OK and OK') 
for the two ends of the cylinder. 

A line through C, perpendicular to PP' fixes the crank posi- 
tion for crank end admission, OA'. 

The steam laps on the two ends are determined by the radii of 
the arcs, s and s', drawn with as a center and tangent to AC 
and A'C respectively. 

The crank eiid lead, L' , is found by erecting a perpendicular 

3 



34 



VALVE GEARS 




Fig. 37. — Solution by Bilgram diagram. 



i Steam Lap =5. 
Lead = L and L . 

Cuf-o-Ff. = C at 6e% of. the Stroke. 

E'xhaust Lap = e andei 




Fig. 38. — Solution by Zeuner diagram. 
Problem D. 



USE OF VALVE DIAGRAMS 35 

from the point P' to the horizontal axis and measuring the 
distance from the foot of this perpendicular to the steam lap 
arc, s'. 

Draw the two circles with PO and P'O as diameters. 

PROBLEM D 

- Given. — Steam lap, head end = s inches 

Leads, both ends = L and L' inches 

Cut-off, head end • = at 69% of the stroke 

Exhaust lap, both ends = e and e' inches. 

Bilgram Solution (Fig. 37) for Problem D. — Draw a pair of 
axes intersecting at and a circle with any arbitrarily assumed 
radius, OD, to represent the path of the crank pin. The stroke 
of the piston is then represented by the diameter of the circle, 
DD'. 

The point m., located at 69% of DD', represents the position of 
the piston when head end cut-off occurs and the corresponding 
crank position, OC, is found by swinging m on an arc whose 
radius represents the connecting rod length (here taken as three 
times the stroke, DD'), to the crank pin circle. 

Draw the head end lead line parallel to the horizontal axis and 
at a distance L (head end lead) from it. Draw the crank end 
lead line in the same manner. 

Draw FH and IJ parallel to the head end lead line and OC 
respectively and at a distance s (head end steam lap) from them. 
The intersection of FH and IJ locates Q, and the head end lap 
circles, s and e, can now be drawn. 

The crank position for head end admission, OA, is tangent 
(when extended) to the circle s. The crank position for head end 
cut-ojf, OC, is also tangent to this circle. 

The crank position for head end release, OR, is tangent to the 
exhaust lap circle, e, and the crank position for head end com- 
pression, OK, is also tangent (when extended) to this circle. 

Draw QO. This determines the angle of advance, 8, and the 
eccentricity, r. 

The center of the crank end lap circles is at Q' , on the circle of 
radius r and diametrically opposite Q. 

The crank end steam lap, s', is determined by the radius of the 
crank end lap circle drawn with Q' as a center and tangent to the 
crank end lead line. 



36 VALVE GEARS 

The crank positions for crank end admission, OA', and crank 
end cut-off, OC, are tangent to the steam lap circle s'. 

Draw the crank end exhaust lap circle about Q' and with the 
given radius e' inches. 

The crank positions for crank end release, 0R\ and compression 
OK', are drawn tangent to the exhaust lap circle, e'. 

Zeuner Solution (Fig. 38) for Problem D. — Draw the axes 
through 0, scribe the circle to represent the path of the 
crank pin and locate the crank position for head end cut-off, OC, 
as explained above in the Bilgram solution. 

With as a center, strike the head end steam lap arc with radius 
s, and the head end and crank end exhaust lap arcs with radii 
e and e', respectively, and lay off the head end lead, L, as 
shown. 

Draw the line HP perpendicular to the horizontal axis from the 
point found by laying off the lead to the left of the steam lap; 
draw FP perpendicular to the crank position OC, from the 
point where the steam lap arc intersects OC. The intersection 
of HP and FP locates P and determines the angle of advance, 
8, and the eccentricity, r. 

Draw PP' and the circle of radius r to represent the path of 
the eccentric center. 

Draw CA perpendicular to PP' and tangent to the head end 
steam lap arc, s, thus locating the crank position for head end 
admission, OA. 

Draw RK and R'K' perpendicular to PP' and tangent to the 
exhaust lap arcs, e and e', respectively, thus locating the crank 
positions for release {OR and OR') and compression (OK and OK') 
for both ends of the cylinder. 

Erect a perpendicular from P' to H' on the horizontal axis and 
lay off the crank end lead, L', as shown, thus determining the 
radius s' of the crank end steam lap arc. 

Draw C'A' perpendicular to PP' and tangent to the steam lap 
arc, s', thus locating the crank positions for crank end admission, 
OA', and cut-off, OC. 

Draw the circles with PO and P'O as diameters. 

PROBLEM E 

Given. — Crank positions for 

cut-off, head end, OC 
compression, both ends, OK and OK' 
Maximum port opening, head end, G inches 
Lead, both ends, L and L' inches. 



USE OF VALVE DIAGRAMS 37 

Bilgram Solution (Fig. 39) for Problem E. — Draw a pair of 
axes intersecting at 0. 

Draw the given crank positions OC (head end cut-off), OK 
(head end compression) and OK' (crank end compression). 

Strike an arc with center at and radius equal to the maximum 
port opening, G. 

Draw the head end lead line parallel to the horizontal axis 
and at. a distance L (head end lead) from it. Draw the crank 
end lead line in the same manner. 

Draw a circle tangent to crank position OC, the maximum 
port opening arc and the lead line. The center of this circle will 
be on the bisector of the angle between OC and the lead line. 
The radius is easier to obtain by trial than by geometrical solu- 
tion. The radius is the head end steam lap, s, and the center 
locates Q, thus determining the angle of advance, 5, and the 
eccentricity, r. 

Draw the eccentric circle with radius r. 

The center of the crank end lap circles is at Q' , on the circle of 
radius r and diametrically opposite Q. 

The crank end steam lap, s' , is determined by the radius of 
the crank end lap circle drawn with Q' as a center and tangent 
to the crank end lead line. 

The crank position for crank end cut-off, OC , is tangent to 
the circle, s' . 

The exhaust lap circles are tangent to the crank positions 
for compression, OK and OK' , for the head end and crank end 
respectively. 

The crank positions for release, OR and OR' , are tangent to 
the exhaust lap circles e and e' . 

Zeuner Solution (Fig. 40) for Problem E. — Draw a pair of 
axes intersecting at Oi and draw the line OiC\ representing the 
angular position of the crank for head end cut-off. 

Anywhere on the line OiCi but the known distance G inches 
(the maximum port opening) apart, locate the points Xi and x^. 
Likewise locate the points yi and y^ anywhere on the horizontal 
axis through Oi but the known distance G-L inches (the maximum 
port opening minus the lead) apart. 

Erect perpendiculars to 0\Ci at Xx and x^ and also perpen- 
diculars to the horizontal axis at yi and 1/2- These perpendiculars 
intersect at W and P. 

Draw a circle through P and tangent to the lines Wx2 and Wy^. 



38 



VALVE GEARS 



A' ^ .L 




Fig. 39. — Solution by Bilgram diagram. 



Given ■• 

Cut-off C. , 

Compression KandK. 
Max.PortOpeninq G. ^ 
Lead L and L . 




Fig. 40. — Solution by Zeuner diagram. 
Problem E. 



USE OF VALVE DIAGRAMS 39 

This circle whose center is at 0, represents the path of the ec- 
centric and determines the valve travel. The point is the real 
center of the diagram, Oi being merely a trial center. 

The completion of the diagram involves only the construc- 
tions explained in Problems A and C. The final crank position 
for cut-off OC is drawn parallel to the trial position OiCi. 
The intersection of OC and PXi determines the head end steam 
lap arc of radius s. Draw AC to locate the crank position OA 
for head end admission. 

Draw the circle of radius L' (the crank end lead) with D' as 
a center and then draw A'C parallel to AC thus determining 
crank end admission OA', cut-off OC and steam lap s'. 

Draw the given crank positions for compression, OK and OK', 
and determine the crank positions for release, OR and OR', and 
the exhaust laps, e and e', as previously explained and as evident 
from Fig. 40. 

Draw the circles with PO and P'O as diameters. 

Comparison of Bilgram and Zeuner Diagrams. — It is interest- 
ing to note that if lines are drawn in the Bilgram diagram 
parallel to QQ' and tangent to both sides of the lap circles, a 
part of the Zeuner diagram results; if the two pairs of Bilgram 
lap circles are then moved in until their centers coincide with 
the intersection of the axes the Bilgram diagram will be con- 
verted into a Zeuner. 

PROBLEM 9 

If the angle of advance remains unchanged, what effect will increasing the 
steam lap have on admission, cut-off and maximum port opening? 

PROBLEM 10 

If the angle of advance remains unchanged, what effect will increasing 
the exhaust lap have on release and compression? 

PROBLEM 11 

Given. — Steam lap 7/8" 

Width of maximum port opening If" 

Valve travel 5\" 

Lead 1/16" 

Length of connecting rod 3 times stroke of piston. 

Draw a Bilgram diagram and indicate the angles through which the crank 
is turning while the port is being opened and while it is being closed. 



CHAPTER IV 
PORT OPENINGS AND PASSAGE AREAS 

One of the most important considerations in successful engine 
designing is to provide the correct passage areas for the steam. 
If the cross-sectional area of the passage-way is too small, the 
steam is throttled, its velocity is increased and its pressure falls; 
this is commonly called wire drawing. On the other hand if the 
passage-way is made too large the clearance volume is un- 
necessarily increased and the economy of the engine is lowered. 
Valves should be designed so that they will give the correct 
port opening. Although some throttling of the steam is unavoid- 
able, it can be reduced by quick opening and closing of the valve. 

The quantity of steam flowing into the engine cylinder at 
any instant while the port is open depends on the area and 
velocity of the piston. During the stroke of the piston its 
velocity, Vp, varies from zero to a maximum and then to zero 
again. It can be shown mathematically that the velocity of the 
piston at any time during the revolution is equal to 

•rr / • o , sin 2d\ 
V,= Vc\^md + -^;^) [1] 

in which Vp = velocity of the piston in feet per minute ; 

Vc = velocity of the crank pin in feet per minute; 
6 = the angle through which the crank has moved 

from the head end dead center ; 
n = ratio between the length of the connecting rod and 
the length of the crank. 

Assuming that n = 6, as is usual practice, the value of the paren- 
thesis for different crank angles, 6, is given in Table 1. 

As long as there is no expansion after passing the port opening 

aVp = a' Vs [2] 

where a = area of the piston, 

a' = area of the port opening, 
Vs = velocity of the steam in feet per minute. 

40 



PORT OPENINGS AND PASSAGE AREAS 
Taulk 1 



41 



6 


0° 


7.. 5° 


15° 


30° 


45° 


00° 


7.5° 1 90° 


sin e 





.13 


.259 


.500 


.707 


.866 


.906 


1.000 


sin 20 
12 


.022 


1 
.042 .072 .083 


.072 


.042 





sin 2e 
sin 9 + — j2~ 





.152 


.301 


.572 


.790 


.938 


1.008 


1.000 



e 


180° 


lS7i° 


195° 1 210° 225° 


240° 255° 


^ 270° 


sin e 





-.131 


-.259 


-.500 


-.707 


-.866 -.966 


-1.000 


sin 2e 
12 





+ .0216 


+ .0417 


+ .0722 


1 i 
+ .0834' + .0723 +.0417 





sin 26 
sm e + j2 





-1.0841 


-.2173 


-.4278 


-.6236 '-.7938 


-.9243 


-1.000 



Combining equations [1] and [2] 



^;-^(sin.+ 



2n 



[3] 



It has been the practice in designing valves to base the com- 
putation for the maximum port opening on a steam speed of 6000 
to 8000 feet per minute when cutting off at about 60% of the 
stroke, and then to use instead of the actual velocity of the piston, 

sin 2d \ 2LN . 

— ^ — I, the average velocity ..^ it. per minute. 



V. (sin d + 



In other words 



6000 to 8000 = 



2LN 



[4] 



a' 12 

where a' is the area of the maximum port opening, 

L the stroke of the engine in inches, 
N the E.P.M. of the engine, 

a the area of the piston. 
Choosing for illustration an 8" X 10" - 250 R.P.M. engine the 
values inserted in equation [4] give 



7000 = 



50.3 



X 



2 X 10 X 250 



a' ' " 12 

from which the area, a', is 

, 50.3 X 2 X 10 X 250 



[5] 



a = 



12 X 7000 
= 3 sq. in. 

1 The — sign here only indicates a change in the direction of motion. 



42 



VALVE GEARS 



This maximum port opening occurs when the crank has turned 
through half the angle between the admission position and the 
cut-off position. For 0.6 cut-off, the valve is widest open when 
d = 48f°. 

The velocity of the crank pin is 



7. = 



rlO 
12 



X 250 = 655 ft. per minute. 



and the area of the piston is 



a = 



= 50.3 sq. in. 



When the valve is widest open a' = 3 sfq. in. 

Then from equation [3], for cut-off at 60% of the stroke 

50.3 _. / . __ . sin 97.5' 



F. = 



X 655 (sin 48f ° + 



3 '" \ * ' 12 

= 9170 ft. per minute. 

This is the actual speed of the steam through the port at the 
instant of maximum port opening. In equation [4] the value 
6000 to 8000, with a mean of say 7000, is not an actual speed but a 
nominal one. It is not the actual speed because, for convenience, 

the average value ^ was used as the piston speed instead of the 

T. / • , sin 2 e\ 

true mstantaneous value Vc (sm 6 + — ^ — ) 

Fig. 41 has been drawn with cut-off at 60% of the stroke and 
with a width of maximum port opening equal to 0.5", i.e., the 
length of the port was assumed to be 6". Port openings 
taken from the diagram for different values of 6 are given in 
Table 2. These values, together with aVc = 50.3 X 655 = 







Table 


2 















Forward 


0° 


7.5° 


15° 


30° 


45° 


60° 
240° 


75° 
255° 


90° 
270° 


Return.. 


180° 


187.5° 


195° 


210° 


225° 






.183" 


.285"" 


.430" 


.497" 


.460" 


.340" 


.130" 






Area of port opening, a'. . 




1.098 


1.710 


2.58 


2.9822.76 


2.04 


.78 










sin 26 
sm + ^2 


Forward 


.1385 


.176 


.222 


.2705 


.34 


.494 


1.281 


. «' 


Return.. 





.0989 


.127 


.166 


.215 


.288 


.452 


1.281 


Vs 


Forward 


4560 


5800 


7310 


8910 


11,200 


16,280 


42,200 


Return.. 


3260 


4180 


5460 


7090 


9480 


14,900 


42,200 



PORT OPENINGS AND PASSAGE AREAS 



43 




Fig. 41. 



50,000 



40,000 



-0 30,000 



a' 
c. 
KP 

E 


a> 



20,000 



o 10,000 









































































i 
























/ 
























/ 






















^ 


/^ 






















/ 


















forvii 


















— 




^ 


r^ 


^n ^^ 








^ 

























15 


30 45° 60 


75 


90" 


195° 


210" £-£5° £40° 
Crank Angles, 6^. 
Fig. 42. 


-£55° 


£70 



44 



VALVE GEARS 



32,947, inserted in equation [3] give the actual speed of the steam 
(Vs) through the port for any position, 6, of the crank. The 
latter values are given in Table 2 and also plotted in Fig. 42. 
It will be noticed that the rate of flow increases gradually 
until the maximum port opening is reached; then it increases 
more and more rapidly, approaching infinity as the valve closes. 
On account of the angularity of the connecting rod the curve 
for the return stroke does not coincide with that for the forward 
stroke. In fixing the limiting steam speeds, only the forward 

Table 3 



e 


0° 


7.5° 


15° 


30° 


45° 


60° 


75° 


Cut-off at 
40%. 






.23" 


.35" 


.485" 


.488" 


.35" 


.08" 










1.38 


2.10 


2.91 


2.928 


2.1 


.48 






Actual steam speed through 
port, Vs. 





3630 


4720 


6480 9080 


14,730 


69,100 


Cut-off at 
30%. 
e = 31.2° 






.31" 


.41" .50": .41.5" 


.16". 
















1.86 


2.46 


3.00 


2.49 


.96 








Actual steam speed through 
port, 7s. 





2690 


4030 


6290 


10680 












Cut-off at 
20%. 
e= 24 . 6° 






.36" 


.46" 


.48" 


.20" 


















2.16 


2.76 


2.88- 


1.20 














Actual steams peed through 
port, V.i. 





2320 


3600 


6550 


22,200 











stroke need be considered since the speed during the return stroke 
will never be greater. 

Maximum port opening, for .0.6 cut-off, occurs at the points 
marked 6 and 6' corresponding to crank positions 48f° past 
the dead centers. 

Now suppose this process is repeated for cut-offs at 40%, 30% 
and 20 % of the stroke, using the same maximum port opening of 
3 sq. in. in each case. The results are given in Table 3 and 
plotted in Fig. 43. 

On the curve for 40% cut-off the point marked 4 indicates the 
time at which maximum port opening occurs and it will be noticed 
that the ordinates, i.e., the steam speeds, are less than indicated 
by the curve for 60% cut-off which has been reproduced herefrom 
Fig. 42. This is because the valve gives the same maximum open- 
ing in both cases but in the latter the period of inflow of steam is 



PORT OPENINGS AND PASSAGE AREAS 



45 



during a time when the piston is moving slower. The curves for 
the earlier cut-offs at 30% and 20% fall still farther below the 
60% curve and show that with a constant maximum port opening 
the steam speed through the port decreases as the cut-off is made 
earlier. 

It might be suggested that if the steam speed represented by the 
point 6 is allowable through the maximum port opening, the 
points 4, 3, and 2 might be raised to the same amount. But at 
best the steam speed through the port is high, and some drop in 




10 15 20 TS 30 35 40 45 50 55 60 65 70 75 



Crank Angles^(9. 
Fig. 43. 



pressure due to wire drawing results. Throttling causes a 
larger percentage loss of work as the cut-off is made earlier. 
Moreover the loss due to initial condensation is proportionately 
larger for the earlier cut-offs. More steam must flow in to fill the 
space vacated by the steam which condenses and the result is a 
higher speed through the port opening than is represented by the 
curve. In reality the ordinates of the curves should be larger, 
-particularly for the smaller values of 6. The earlier cut-offs 
should therefore be given special consideration. 

Obviously, though, the curves for 40%, 30%, and 20% 
cut-off can be raised somewhat, or in other words the port open- 
ings can be reduced, without the steam speeds becoming too 
high. Points 4, 3, and 2 will not be raised to the same height as 



46 



VALVE GEARS 



6 but the curves on which they lie will be raised so that they will 
approximately coincide with the curve through 6. It is evident 
from an inspection of the curves that it is impossible to make them 
coincide after the point of maximum port opening is passed. 

The curve through point 4 will be raised to approximate the 
curve through 6 by making the ordinates at ^ = 30° coincide.^ 
This can be done by decreasing the values of a' in Table 3 pro- 
portionately. These values were originally obtained by measur- 
ing Fig. 41 which was constructed with a maximum port opening 
0.5" wide. Reducing this port opening in proportion to the 
ordinates at the point 6 = 30° in Fig. 43 gives 



65 

0.5 X Wo 

to 



0.445' 



or the same result would be obtained from equation [5] by 
using instead of 7000 

73 



then 



7000 X ^ = 7860 
65 

7860 ^^X?^'"^ 2^° 



0.445' 



a: 12 

a' = 2.67 sq. in. 

and the width of maximum port opening = -^^ 

That is, the nominal steam speed should be increased as the cut- 
off is made earlier. 

By reconstructing Fig. 41 with a maximum port opening of 
0.445" and cut-off at 0.4 stroke and measuring the port openings 
for various crank angles 6, as before, the following results are 
obtained. Plotting these values of Vg gives the 40% curve in 
Fig. 44. 

Table 4 



e 


7.5° 


15° 


30° 


45° 


60° 


75° 


Width of port open- 
ing. 


0.205" 


0.31" 


0.43" 


0.432" 


0.31" 


0.071" 


Area of port 
opening, a'. 


1.23 


1.86 


2.58 


2.592 


1.86 


0.426 


Actlial steam 
speed, Vs. 


4070 


5330 


7310 


10,260 


16,620 


29,800 



^This ordinate was chosen by inspection and trial. 



PORT OPENINGS AND PASSAGE AREAS 



47 



Raising the curve for 30% cut-off (Fig. 43) in the same manner 
and making the ordinates at ^ = 22|° coincide by using a 

5200 
width of maximum port opening equal to 0.5 X t^ttttk = 0.394' 



6600 



or a nominal steam speed in equation [5] equal to 

6600 



7000 X 



8880 



5200 
gives the curve marked 30% in Fig. 44. 

24,000 
££,000 

i 20,000 
£ 

^ 18,000 
e. 
^ 16,000 

-O 14,000 

J- 12,000 

£ 10,000 
cs 

^ 8,000 
en 

■g 6,000 

t 4,000 

2,000 

5 10 15 20 -£5 30 35 40 45 50 55 60 65 70 75 
Crank Angles, O. 
Fig. 44. 

The curve marked 20 % in Fig. 44 is obtained in a similar way 
by making the ordinates at = 20° in Fig. 43 coincide. This is 
equivalent to computing the maximum port opening from equa- 
tion [5] with a nominal steam speed of 7000 X TTn?; = 10,200 

which gives an opening 0.343" wide. 

The points marked 6, 4, 3, and 2 in Fig. 44 indicate the instant 
of maximum port opening and it is seen that the earlier cut-offs are 
favored by a lower steam speed. 

Equation [4] is in a very simple and convenient form and 
will be used for computing maximum port openings. Instead of 
using the value 7000 for the nominal steam speed, however, this 





























—~^ 




















1 




























i 


6 




1 




o\o 






















1 


^ 


^ 




t 




















/ 




.^ 


\' 


9 




,1° 
















/ 


/ 


A 


/ 


7 




r,»}'^> 


^> 
















/ 


/ 


/ 


^ 




^ 


^ 














? 


^ 


3^ 


t^ 


^ 


6 


^ 
















, -r^* 


^ 


























^ 


^ 


^ 


























/ 


'^ 






























/ 

































48 



VALVE GEARS 



figure will be made to depend upon the cut-off and chosen so as 
to give the curves of actual steam speed shown in Fig. 44. In de- 
veloping these curves the nominal steam speeds were found to be 

for the 20% cut-off curve 10,200 ft. per min. 

for the 30% cut-off curve 8,880 ft. per min. 

for the 40% cut-off curve 7,860 ft. per min. 

for the 60% cut-off curve 7,000 ft. per min. 



1 1,UUU 


1 












































































u 10,000 






























s, 


























\, 




















-1- 






s 




















"S 9,000 








\ 
























\ 


















Q- 










s 


























\ 
















a 8,000 












\ 
























S 














+- 














X 




























■^^ 










i 7,000 


















■-^^ 


»^ 






























o 


























Z 


























cnnn 



























zo 



30 40 50 

Cut-o-ff, percent. 

Fig. 45. 



60 



70 



By plotting these values and drawing a smooth curve (see Fig. 45) 
the nominal steam speed to use in equation [4] for any cut-off 
is determined. 

Nominal Steam Speed 
for Max. Port Opening De- 
pends on Time of Cut-off; |<;>i 
take from Fig. 45 ]'' 




NominahSfeam Speed , 
5000 '/min. --''' 



■■•■ Nominal Steam Speed 
6000 ymin. 



Fig. 46. 



This curve is to be used only as a guide, no rigid rule can be 
given for steam speeds. The valve travel is always an important 
item and must be considered equally with the steam speed and 
the two adjusted to suit each particular case. 

When the exhaust port opens, the cyhnder is full of steam of 



PORT OPENINGS AND PASSAGE AREAS 49 

higher specific volume than when it entered. As soon as the 
port is opened there is a rush to the exhaust cavity and a large 
quantity of the steam escapes before the piston reaches the end 
of its stroke; what remains is pushed out by the piston on the 
return stroke. To insure ample passage for the exhaust steam 
and allow for the roughness and imperfections of castings the 
nominal steam speed through the cylinder ports is usually taken 
as 5000 ft. per minute. The edges of the ports in the valve 
seat are usually smoothed up and through these openings the 
speed may be taken as 6000 ft. per minute. 

A summary of the results of this discussion is given in Fig. 46. 

PROBLEM 12 . 

Design a D-slide valve for a 10" X 12", 200 R.P.M. engine, having 
given the following data: 

Initial steam pressure, 110 lb. gauge. 

Back pressure, 2 lb. gauge. 

Clearance volume, 9% on each end. 

Length of connecting rod three times the stroke. 

Admission on both ends to occur when the crank is 4° before the dead 

center position. 
Head end cut-off at 50% of the stroke. 
Head end compression at 79% of the stroke. 
Crank end compression at 82 % of the stroke. 
Draw the valve and the probable indicator cards. 

ttIO" 

Solution. — The area of the piston is -j- = 78.5 sq. in.; the average 

A ■ ^i'A^ 2 X 12 X 200 ^^^ ,^ • + +, ■ 1 

piston speed is ■ -.^ = ^o — 400 ft. per mmute; the nommal 

steam speed for 50 % cut-off is given in Fig. 45 as 7300 ft. per minute. Sub- 
stituting these values in the expression given below (equation [4] repro- 
duced) determines the maximum port opening. 

Area port X nominal steam speed = area piston X avg. piston speed. 

Are. port - ^^*^ - 4.3 sq. in. 

If the length of the port is assumed to be 3/4 the diameter of the 

cylinder, the length will be 3/4 X 10 = 7.5" and the width of the maxi- 

4 3 
mum port opening ^. = 0.574". 

Bilgram Diagram (Fig. 47). — Take some length DD' to repre- 
sent the stroke of the piston and on this as a diameter draw a 
circle to represent the path of the crank pin. Through the center, 
0, of this circle draw a pair of axes. 

4 



50 



VALVE GEARS 



At an angle of 4° with the horizontal axis draw the crank 
positions for admission, OA and OA'. 

The length DO is 50 % of DD' and the piston has traveled from 
D to when head end cut-off occurs. With the connecting rod 
length as a radius (3 X DD') and center on D'D produced, scribe 




Fig. 47. — Bilgram diagram for Problem 12. 

an arc through cutting the crank pin circle at C and determining 
the crank position OC for head end cut-off. 

With center at and a radius equal to the maximum port 
opening, 0.574", scribe the port opening arc. 

Tangent to the port opening arc, to the crank position at cut- 
off (OC), and to the crank position at admission {OA produced — • 



'C~>- 



PORT OPENINGS AND PASSAGE AREAS 51 

which happens to fall on OA') draw the head end steam lap circle. 
Its radius measures 1.43" and the angle of advance, 8, is 49.3°. 

Lay off D'fn equal to 79% of D'D and with the same radius 
as previously used to represent the connecting rod, scribe the 
arc mK and draw OK the crank position for head end compression. 

With center at Q draw the head end exhaust lap circle tangent 
to OK produced. Its radius measures 0.06". 

The crank position for head end release, OR, is tangent to the 
lower side of the head end exhaust lap circle. By projecting 
R on an arc to the horizontal axis the corresponding piston posi- 

Dn 
tion is located and jyfy gives the percent of the stroke at which 

release occurs. 

^ = 86%. 

Tangent to the head end steam lap circle and parallel to DD' 
draw the lead line. Its distance from the horizontal axis meas- 
ures 0.09". 

Having determined the time of the four events for the head 
end, and having given the initial steam pressure and the back 
pressure the indicator card for that end can be drawn. 

Following a similar construction for the crank end, locate Q' 
diametrically opposite Q and draw the crank end steam lap 
circle tangent to OA' produced. Its radius measures 1.43" 
The line OC tangent to the other side of the circle gives the 
crank position for crank end cut-off. Swing C to the axis and 
determine the percent of cut-off. 

^ - 429^ 
D'D ~ ^^ Z'^- 

Lay off Dm' equal to 82 % of DD' and locate the crank position 
for compression {OK') by projecting m' on an arc to the crank 
pin circle. 

Draw the crank end exhaust lap circle, the crank position for 
release, OR', and the lead line, in the same manner as for the 
head end. 

Draw the crank end indicator card. 

The Solution by the Zeuner Diagram is shown in Fig. 48. 

To draw the valve and seat proceed as follows: 

Draw a line to represent the valve seat (Fig. 49) and on it 



52 



VALVE GEARS 



locate a point x. Lay off xx2 = 1.43", the head end steam lap, 
as determined by the diagram. The width of the port xxi should 
be sufficient to limit the nominal speed of the exhaust steam to 
6000 ft. per minute. 



78.5 X400 
6000 



= 5.23 sq. in. 



5 23 

-jrj = 0.698" or say 0.7" width of port. 




Fig. 48. — Zeuner diagram for Problem 12. 



Multiplying the width of port opening for entering steam by 
the ratio of the nominal steam speeds.gives the same result: 

7300 
.574" X gQQQ = 0.698" say 0.7". 

The wall between the cylinder port and the exhaust cavity is 
called the bridge. For this work the width of the bridge, t, may be 
chosen arbitrarily. It should be at least as thick as the cylinder 
walls and wide enough to give the valve a good bearing surface 
and prevent leakage of live steam into the exhaust when the 
valve is in its extreme position. In this problem it will be as- 
sumed 3/4 ". When the valve is in either of its extreme positions 



POET OPENINGS AND PASSAGE AREAS 



53 



the inside edge closes off some of the exhaust cavity. Fig. 49 
shows the valve dotted in the left hand extreme position. It is 
necessary to make the cavity wide enough so that there will re- 
main sufficient passageway for the exhaust steam — in this 
problem 0.7". 



j<- go/ 

\<—Z.00-—>\<—l.43"->\ ^K- 0.06" 



0./8"->\-\<- \<—l.43"->^ 




Valve Seat 



-■^—f-^oJ-X 



Voi-^-t-^ C-- 

At least O.7"+2.00"-tOJ8"'-t^say 2'^" 
Fig. 49. 



In Fig. 49 

C + i = e' + r + 0.7" 

C = e' + r + 0.7" - t 

= 0.18" + 2.00"+0.7"- 0.75" = 2.13" say 2i". 

The expression shows that the larger exhaust lap should always 
be used in this computation. 




■Angularity of connecting rod. 



Draw the crank end cyHnder port and the crank end of the 
valve with the laps determined by the diagram and finally fix 
the finished length of the valve seat so that the valve will not 
wear shoulders at the ends of its travel. 

Angular Movement of Connecting Rod. — When the piston has 
moved forward to the middle of its stroke as from A to in 
Fig. 50 the crank has not turned through 90° but only through the 
angle AOF. During the second half of the stroke of the piston 



54 



VALVE GEARS 



the crank pin moves from F to A\ Assuming the velocity of the 
crank pin to be uniform, the piston speed is greater during the 
first half of the forward stroke (A to 0) than during the second 
half (0 to A'). The amount of variation of the piston speed 
from the simple harmonic depends on the relative lengths of the 
crank and connecting rod or rather on the angle which the con- ' 
necting rod makes with the line of stroke. This movement of the 
connecting rod through the angle bisected by the line of stroke 
is termed its angular vibration. Its angular position at any 



1,000- 
900 



800- 



700- 



600 



V. 500- 



(O 400- 

c 
o 
t 300 



200- 



100- 



-^-7 
o 

c 



> ^ 

-H^-4 

c 
o 

(0 -^ 



















(."■ 


ss^M 






















A^ 


.f 
























f 


















.n 


1 




K 


















if 


/ 1 


1 

1 
1 


'A 


1 ^ 
1 1 


\ 
















'\ 




1 


A 


{ r 
] 1 
] 1 


V 


\ 












1 


1 
r 
1 
1 




A 




1 j 




\ 


\ 










/ 


1 
1 
1 

1 


'A 


1 




1 1 






1 \ 
1 \ 

1 V 








/ 


' 


V 


f 1 


1 
1 

1 




1 1 
1 1 






1 

1 


\ 






I 


y 


1 
1 




1 

1 










1 




\ 




t 


S 3 


4 
t 


5 6 
Cra" 


;7 

nk '/^ 


5 1 9 
^ngl 


1^ 

es,C 


D5 t 


10 \l 
aes . 


55 15 


JO I 


&5 \i 


50 



10 20 30 40 50 60 70 80 
% S-t-roke. 
Fig. 51. 



90 



100 



time is termed its angularity or obliquity. The effect of 
angularity is to draw the pistgn closer to the crank shaft than if 4> 
were zero. 

A curve of piston displacements for different crank angles is 
shown in Fig. 51. The ratio between connecting rod length and 
crank length used in this figure is 6 to 1 and the stroke of the 
engine is 10". It will be noticed that the curve is not exactly 
symmetrical about the line 5 representing half stroke and that 
to be symmetrical it must pass through the point A. 



PORT OPENINGS AND PASSAGE AREAS 



55 





T 


ABLE 5.— STROKE TABLE. 






Piston 
position 
(stroke = 

unity) 


Crank angles (for ordinary connecting rod.s) | 


Ratio crank to connecting rod 1 to 5 || Ratio crank to connecting 


rodl to 6 


Forward 


Return 


Diff. II Forward | Return 


Diff. 


Deg. 


Deg. 


Deg. II Deg. | Deg. 


Deg. 


0.025 


161 


20J 


3f 


161 


191 


3 


0.05 


23-1 


281 


41 


24 


28 


4 


0.075 


29i 


35i 


61 


291 


34f 


sr 


0.10 


33 1 


40f 


7 


341 


391 


61 


0.125 = i 


371 


45f 


7f 


381 


44f 


6i 


0.20 


48f 


58i 


91 


491 


57J 


7f 


0.25 =i 


551 


651 


10 


56i 


641 


81 


0.30 


6U 


72 


m 


62 1 


71 


81 


0.333 = i 


651 


761 


101 


66i 


75i 


9 


0.375 = 1 


70| 


81f 


111 


711 


80i 


91 


0.40 


73 


84i 


iii 


731 


831 


91 


0.45 


781 


90 


111 


79f 


881 


n 


0.50 =i 


84| 


95f 


lit 


851 


94| 


9i 


0.55 


90 


lOlf 


111 


91| 


loot 


n 


0.60 


95f 


107 


lU 


96f 


1061 


91 


0.625 = 1 


981 


1091 


111 


99f 


1081 


91 


0.65 


lOlf 


1121 


101 


1021 


lllf 


9 


0.666 = f 


1031 


1141 


101 


104| 


1131 


9 


0.68 


1051 


116i 


lOf 


106§ 


1151 


81 


0.70 


108 


1181 


lOi 


109 


1171 


81 


0.71 


1091 


119f 


lOf 


not 


118f 


81 


0.73 


112 


122i 


101 


113 


1211 


8f 


0.75 =f 


1141 


124f 


10 


1151 


123f 


81 


0.76 


1161 


1251 


9f 


117 


1251 


81 


0.77 


1171 


127i 


91 


nsi 


1261 


8 


0.78 


119 


1281 


9^ 


120 


1271 


7f 


0.79 


120^ 


129f 


91 


1211 


129 


71 


0.80 


1211 


131i 


n 


1221 


130^ 


71 


0.81 


123i 


1321 


9 


1241 


131f 


7f 


0.82 


125 


1331 


8f 


126 


1331 


71 


0.83 


1261 


1351 


8f 


127§ 


134i 


7 


0.84 


128i 


1361 


81 


1291 


136 


61 


0.85 


130 


138i 


Si 


1301 


137J 


61 


0.86 


131f 


1391 


81 


1321 


139 


6* 


0.87 


1331 


141i 


7f 


134i 


1401 


61 


0.875 = 1 


. 1341 


142i 


7f 


135i 


1411 


6i 


0.88 


135i 


1421 


7f 


136i 


1421 


61 


0.89 


1371 


144i 


7i 


138 


144 


6 


0.90 


139i 


146i 


7 


1401 


1451 


51 


0.91 


1411 


148 


61 


1421 


147i 


51 


0.92 


1431 


1491 


6i 


1441 


1491 


5 


0.93 


146 


1511 


51 


1461 


15U 


4| 


0.94 


im 


154 


5i 


1491 


1531 


41 


0.95 


15U 


1561 


4^ 

*8 


152 


156 


4 


0.96 


154i 


1581 


4f 


1541 


1581 


31 


0.97 


1571 


161| 


4 


1581 


16H 


3i 


0.98 


1611 


1651 


3i 


162i 1641 


21 


0.99 


1671 


im 


2f 


1671 1691 


2 



56 



VALVE GEARS . 



Overtravel. — Valves are sometimes made to overtravel the 
cylinder port so that in the extreme position the valve stands as 
shown in Fig. 52. The valve could give a maximum port opening 
equal to G inches but the width (p) of the port is sufficient for the 
passage of the steam. 

Valve with 

Overtravel '. 




Fig. 52. — Overtravel. 




w 



Valve without ■ 

Overtravel-' 



Fig. 53. — Valve displacement curves. 



The displacement of the .valve from its admission position 
plotted against intervals of time as abscissae gives the curves 
shown in Fig. 53. Rapid opening and closing of the valve will 
produce lines at the sides of the half ellipse which are more nearly 
vertical than when the action is sluggish. Consequently such 
curves offer a satisfactory means of comparing different valves. 




Fig. 54. — Bilgram diagram. 



Fig. 55. — Zeuner diagram. 



The purpose of overtravel is to produce a quick valve action 
by obtaining valve curves as shown by a in Fig. 53. Without 
overtravel the curve is similar to 6. Whether the valve is made 
to overtravel or not, the distance G in Figs. 54 and 55 is the dis- 
tance from the farthest edge of the port to the edge of the valve 
when the valve is widest open. 



PORT OPENINGS AND PASSAGE AREAS 57 

PROBLEM 13 

Given. — Head end steam lap, Is" 
Crank end steam lap, If" 
Angle of advance, 50° 
Width of cylinder port, |" , 
Eccentricity, 2" 
Head end exhaust lap, xe" 
Crank end exhaust lap, |". 

Draw the diagram for the valve (either Zeuner or Bilgram), show the 
steam distribution by indicator cards, test for overtravel, and for com- 
pleteness of opening for both inlet and exhaust. 



CHAPTER V 
FORMS OF SLIDE VALVES 

Piston Valves. — There are two general types of slide valves 
—flat valves and piston valves. Each type is represented by 
many different styles. 




K--s->i -Hik-g 



Fig. 56A. 




^^^^^^^ 




->|:K- l<-S--^>j 



|<-S'->1 ->ie'K- 



FiG. 56C. 



The D-shaped valve shown in Fig. 56A is the simplest form of 
the fiat slide valve. If the D valve is rolled up into the form of a 
cylinder it becomes the simplest form of piston valve (Fig. 56B), 

58 



FORMS OF SLIDE VALVES 



59 



Usually with flat valves the live steam surrounds the valve 
and the exhaust passes into the cavity E (Fig. 56A) while with 
piston valves (Fig. 56C) the live steam enters the cavity S and the 
exhaust passes out at the ends of the valve to chambers E~E. 

,, , .Bushinq 

Valve. ; ^ 




Inlet 



Exhaust 




Fig. 57. — Cylinder and valve of Ideal engine 



The steam and exhaust laps for the two cases are shown in the 
figure. 

Piston valves are extensively used in marine work. They are 
also used on locomotives and on stationary high speed engines. 




Fig. 58. — Ideal engine. 

An example of the last mentioned application is given in Fig. 
57, which shows the cylinder, valve and valve chest of the Ideal 
Engine, built in Springfield, 111. Steam enters past the inside 



60 VALVE GEARS 

edges of the valve and exhausts past the outside edges. The 
exhaust passes out from either end of the steam chest into a 
Y-shaped casting and then to the exhaust pipe as shown iii 
Fig. 58. The valve and bushings are shown in. Fig. 59. 

A common form of locomotive piston valve is shown in Fig. 
60 with a few dimensions marked to give an idea of the size and 
proportions. The two end pieces and the central body are held 
together by the valve rod. Each end piece secures a bull-ring 
which carries two spring rings. These' rings are turned to a 
larger diameter than the bore of the valve cylinder and are 
then split so that they can be sprung into place. They form 
the working faces at each end of the valve. Any wear allows 
the rings to automatically spring out and keep the valve tight. 




,'L/ve Steam Port 



''Exhaust Steam 
Port 



Fig. 59. — Piston valve and bushings. 

Some idea of the form of the cylinder casting for a locomotive 
with a piston valve may be obtained from Fig. 61. Particular 
attention is called to the grid-bushings inserted at each cylinder 
port, and to the form of the passageways leading the exhaust 
to the base of the stack. 

The advantages of the piston valve are: 

1. It is perfectly balanced against steam pressure. 

2. It is light. 

3. By taking steam inside, it relieves the valve rod stuffing 
box and the valve chest end plates of high pressure steam so 
that they are easily kept steam tight. 

The disadvantages are: 

1. It gives larger clearance than a flat valve. 

2. It usually requires a rather complicated cylinder casting. 

3. Any wear results in increased leakage unless the valve is 
made with spring rings or some other means of adjustment. 



FORMS OF SLIDE VALVES 



61 




62 



VALVE GEARS 







FORMS OF SLIDE VALVES 



63 



With an outside admission valve, when the crank is in the head 
end admission position as shown in Fig. 62, the eccentric is at 
a and the valve is to the right of its mid-position the distance 
a". With an inside admission valve, when the crank is in the 
head end admission position (Fig. 63) the eccentric is at a and 
the valve is to the left of its mid-position the distance a". 
This does not affect the construction of the valve diagram. 

The terms direct and indirect are often -used as synony- 
mous with outside and inside admission. A direct valve is 
one having live steam at the two ends and exhaust between 
(see Figs. 56A and 56B). An indirect valve is one having ex- 
haust steam at the two ends and live steam between (Figs. 56C 
and 57). 





Fig. 62. — Outside admission. 



Fig. 63. — Inside admission. 



Fig. 64 shows the relative positions of the crank, eccentric 
and valve of an engine equipped with an outside admission valve 
and turning over. Below it is another figure (Fig. 65) showing 
an engine with the eccentric set to turn over but with an 
inside admission valve. Figs. 66 and 67 are similar to the ones 
mentioned above except that the engine in both cases is 
shown to turn under. To the right, small valve diagrams are 
drawn to show the location of the valve circles in the different 
cases. 

Multiple Ported Valves. — In all the problems which have been 
discussed, only the D or the simple piston valve has been con- 
sidered. Inspection of the figures on pages 50 and 53 shows that 
if a D-slide valve is used for early cut-offs the steam laps and 
the valve travel become very large so that a large, clumsy valve 
results. Usually, too, a valve of this kind has full steam pressure 



64 



VALVE GEARS 



Outside Admission Valve. 
Bngine Turning "Over!' 




Fig. 64. 




Inside Admission Valve. ~A 
Engine Turning "Over." 




Fig. 65. 




Outside Admission Valve . K' 
Engine Tumi rig "Under.'" ! 




Fig. 66. 




Inside Admission Valve. 
Engine Turning "Under, " 




Fig. 67. 



FORMS OF SLIDE VALVES 



65 



on its entire back, pressing it against its seat and making it 
hard to move. As the cut-off is made earher, the angle of 
advance becomes larger and the opening and closing of the 
cylinder port more sluggish. These disadvantages practically 
prohibit the use of the D-slide valve for early cut-offs. 




'•'^2ai-c 



Fig. 68. — Allen valve in mid-position. Fig. 69. — Allen valve admitting 

steam. 

To reduce the valve travel, more than one passage for the 
entering steam can be provided. The Allen valve shown in 
Figs. 68 and 69 is an example of a double ported valve. Fig. 68 
shows the valve in mid-position and Fig. 69 shows it moved to 
the right and steam entering the cylinder. 




WZ^SZ22M2Zm 




Exhaust 



Fig. 70. — Double-ported piston valve 



Certain relations between the dimensions of the valve and seat 
must exist for the two steam passages to open at the same time. 
It will be seen that this valve gives two openings to steam for the 
same amount of movement that the plain valve requires to give 
one opening. Or, in other words, a double-ported valve gives the 
same opening to Kve steam as a single-ported valve, with only half 
the movement. 

5 



66 



VALVE GEARS 



Pressure Plate 




Fig. 71. — Sections through Ball engine cylinder. 




Pressure plate. 




Four-ported valve. 




"^^ Pressure 
Plate 



Length of Port 7." 
Valve for 8"xlZ"-200 R, p.m. Engine. 
Fig. 72. — Four-ported valve and pressure plate. 



FORMS OF SLIDE VALVES 



67 



The Allen idea has been applied to piston valves as illustrated 
by the indirect, double-ported valve shown in Fig. 70. 
The cylinder and valve of the Ball engine built in Erie, Pa., are 




Fig. 73. — Ball and Wood telescopic valve. Double-ported, inside admission. 



SPHERICAL WRIST PINS 




Fig. 74. — Skinner simple engine. 

shown in Fig. 71. Valves of this type are known as double-ported 
flat valves. 

Flat valves can be made four-ported as illustrated in Fig. 72. 



68 



VALVE GEARS 




'bil 

PI 

Pi 
pi 






FORMS OF SLIDE VALVES 



69 



Four-ported valves require only one-quarter the movement of 
single-ported valves to give the same amount of port opening. 

The purpose of the press^tre plate used with these valves is to 
reduce the friction of the valve by relieving the pressure on its 
back. The recesses in the pressure plate form part of the 
passage-way for the steam. 

A flat valve working under a pressure plate possesses some of 
the desirable features of the piston valve with the additional 
advantage of easier repair after it has become worn and leaky. 
To take up wear it is only necessary to plane off the face of the 
valve and then reduce the distance pieces which hold the pres- 
sure plate off the valve until a working fit is obtained, 

A double-ported telescopic valve made by the Ball and Wood 
Engine Company of Elizabeth, N. J., is shown in Fig, 73; this 










Fig. 76. — Skinner engine valve. 



valve is made in two parts, one fitting snugly within the other. 
Steam enters through the top of the valve and tends to separate 
the two parts thus keeping them pressed firmly on the seats, 
reducing leakage and taking up wear. The objections to this 
valve, in addition to the fact that it is not balanced, are its size, 
the large cylinder clearance due to the complex ports, and the 
difficult cylinder casting. 

Summary. — The travel of a valve may be reduced by increasing 
the number of passages for the steam. 

The friction of the valve may be reduced by balancing. the 
steam pressure on the valve. 

Both of these reductions reduce the stresses on the gear. 

Exercise. — Consulting any available references, draw and 



70 



VALVE GEARS 



describe the action of some slide valve and gear not described in 
this book. 

Skinner Engine Valve. — Fig. 74 shows the Skinner simple 
engine, Fig. 75 a longitudinal section of the Skinner compound 
engine and Fig. 76 the valve. As far as steam distribution is 
concerned the action of the valve is similar to that of a D-slide 
valve but the Skinner valve is made in two parts, one pressing 
against the valve seat and the other against the valve chest 
cover. About 80% of the surface of the valve is thus relieved of 
steam pressure. The portion in contact with the cover is called 
the balance ring and is pressed against the cover by coil springs. 
Leakage between the ring and the hub of the valve is prevented by 
spring packing rings. 

Location of Ports in the Valve Seat. — Friction and weight are 
important considerations in all valve gear work. It requires work 
to move the valve against friction, and work to accelerate and 
retard the moving masses. The greater the forces acting, the 
greater will be the wear and tear on the gear. 



Valve in Extreme Position -. 




C -i-t ^p i- r + e' 
c ^ p + r + e'-t 

Fig. 77. 

The D-slide valve usually has steam pressure over its entire 
back and the friction and weight are proportional to the size of the 
valve. It is advisable therefore to make the valve small and this 
requires the ports in the valve seat to be as close together as 
practicable. When the valve is in its extreme position the ex- 
haust passage must not be cramped, otherwise the back pressure 
will cause a loss of effective work. The minimum value of the 
dimension c in Fig. 77 will leave a passage p inches wide for the 
exhaust when the valve is in its extreme position, p being the 
width of the cylinder port through which the exhaust passes. 
Making the ports in the valve seat as close together as possible, 
or in other words making the dimension c a minimum, results in 



FORMS OF SLIDE VALVES 



71 



long passageways in the cylinder and large clearance space; this, 
however, is regarded as the lesser of two evils. 

Balanced valves have not the disadvantage of large friction 
due to steam pressure but 
they produce inertia forces. 
Por such valves the distance c 
is increased so as to reduce the 
clearance. It then becomes 
a matter of judgment how 
much to reduce the clearance 
by increasing the size and 
weight of the valve. Ideas 
regarding the practice in pro- 
portioning the valve and ports 
can be obtained by observing modern engines of standard make. 

Sliders and Rockers. — Many different styles of connections 




Fig. 78. — Slider on Ideal engine. 




Fig. 79. — Typical slider connection. 



are used between the eccentric rod and the valve stem on slide 
valve engines but all can be grouped under the general types: 

(a) Sliders. 

(b) Rockers. 



72 



VALVE GEARS 



Characteristic examples of slider connections are shown in Figs. 
78 and 79. Representative rocker connections are shown in Figs. 
74, 75 and 80. Rockers are used whenever the line of action of 
the valve rod is at a considerable distance from that of the eccentric 




Fig. 80. — ^ Angle compound engine. {American Engine & Electric Co.) 

rod, as when the eccentric connection is placed outside of the 
flywheel. The practice of locating the governor and valve gear 
outside of the flywheel, where the parts are all accessible, has 
become very common in recent years. 



CHAPTER VI 
SHAFT GOVERNORS 

All engine governors are constructed so that certain forces are 
in equilibrium at the desired or normal speed of rotation and a 
disturbance of this equilibrium, due to a change of speed, causes 
the governor to readjust itself. The motion of the governor 
in assuming its new position alters the steam distribution so that 
the speed returns to normal and the governor to its standard 
position. 

The practical application of the shaft governor dates back 
to about 1875 and since that time its development in this country 
has been rapid. In England and continental Europe its use is 
much more limited. 

Shaft governors rotate with the flywheel in a plane perpendicu- 
lar to the axis of the shaft. The position of the governor is 
dependent on centrifugal and inertia forces acting in opposition 
to the pull of one or more springs attached to the wheel on which 
the governor is mounted. 

In all forms of shaft governors the eccentric is under absolute 
control of the governor; it is fastened to the governor so that 
any movement of the latter causes a displacement of the ec- 
centric center. This shifting of the eccentric results in a change 
in the cut-off, thus proportioning the supply of steam to the load. 
Equilibrium exists only at a certain fixed speed and that speed 
can be altered by varying the spring tension or the weights of the 
governor. Additional weight causes the engine to run slower; 
tightening the spring will speed it up. A combined change may 
not affect the average speed but will affect the regulation. 

The movement of the governor may cause 

(a) A change in the angle of advance. 

(6) A change in both the angle of advance and the radius of the 
eccentric. 

The latter is much more common. 

Shaft governors may be divided into two classes: 

(a) Centrifugal governors. 

(b) Inertia governors. 

73 



74 



VALVE GEARS 



These classes have been named after the predominant force acting 
to move the governor. In all centrifugal governors there are 
some inertia forces, and in all inertia governors some centrifugal 
forces, acting on the movable parts. As examples of the first 
class the Westinghouse governor, illustrated in Fig. 81, and the 
Buckeye governor (Fig. 82) have been chosen. 

Westinghouse Governor (Fig. 81).- — In the Westinghouse 
governor a disk, A, on the engine shaft serves as a support for the 
governor parts; this disk may be replaced by the ordinary fly- 
wheel. The weights, BB, are pivoted to the disk at h and h. 




Earliest Cut-off. 

Fig. 81. 



Latest Cut-off. 
-Westinghouse governor. 



The eccentric, C, is rigidly connected to the arm c, which is 
pivoted to the disk at d. Springs DD are attached at one end to 
the weights BB, and at the other (by a lug bolt) to the disk A. 
The long link, e, connects the two weights in such a way that they 
always move together. The short link, /, is attached at one end 
to the eccentric and at the other to one of the weights so that as 
the weights move out, the eccentric moves across the shaft. 
Stop pins, ss, limit the outward travel of the weights. 

The direction of rotation is shown by the arrow. The. distance 
from d to the center of the eccentric is greater than the distance 
from d to the center of the shaft by an amount approximately 
equal to the lap of the valve. As the eccentric is drawn toward 
the right from the position shown in the left-hand figure its center 
is moved farther from the center of the shaft, the valve travel is 
increased and the angle of advance is decreased. The increased 



SHAFT GOVERNORS 



75 



valve travel combined with the decreased angle of advance causes 
a later cut-off without materially altering the point of admission. 

Buckeye Govemor (Fig. 82). — The Buckeye governor is 
somewhat similar to the Westinghouse, the main difference 
being that as the weights move out the eccentric sheave is rotated 
about the shaft center 0. This varies the angle of advance but 
not the eccentricity. 

An essential feature in both these governors is that the weight 
arms are pivoted on a diameter and connected in such a manner 
that their movement with respect to the shaft center is sym- 
metrical, thus securing revolving balance. 




Fig. 82. — ^Buckeye governor. 



Inertia forces which are necessarily present in centrifugal 
governors may have considerable effect on the action of the 
governor. If the governor parts are so arranged that the inertia 
force acts to assists the centrifugal force, the action of the 
governor is made quicker. The direction of the principal inertia 
forces for the extreme positions of the Westinghouse and the 
Buckeye governors are shown in Fig. 83. It is noticed that they 
pass very close to the pivots so that their turning moments are 



76 



VALVE GEARS 



small, less in the case of the Buckeye than the Westinghouse. 
This torque in the Buckeye is negative for some positions and 
hinders the action of the governor. 

PmtP/n^. 




Westinghouse governor Buckeye governor 

Fig. 83. — Directions of principal inertia forces. 

Rites Governor. — The governor shown in Fig. 84 was invented 
by Mr. F. M. Rites and is very extensively used on high speed 
engines. It illustrates, admirably, the inertia type of governor. 




Fig. 84. — Rites inertia governor. 

A heavy arm with weights at the ends is pivoted to the flywheel at 
the point P. The center of gravity of the entire mass is at G 
and the centrifugal force of the entire mass, imagined concentrated 



SHAFT GOVERNORS 



77 



at the point G, causes the governor to assume a certain position 
Umited by the pull of the spring. Should the engine tend to 
slow down, due to increased load, the inertia of the mass of iron 
causes the arm to rotate about the point P and shift the eccentric 
so that the cut-off is made later. More steam is thus admitted 
per stroke. If the engine should tend to speed up, due to de- 
creased load, the arm would lag behind the shaft and turn about 
the pivot P, so that the cut-off would occur later. 

Robb -Armstrong Governor. — In the Robb- Armstrong gov- 
ernor, illustrated in Fig. 85, the centrifugal force is high and to 
this is added a powerful linear inertia. 




Fig. 85. — Robb-Armstrong governor. 

Both the Rites and the Robb-Armstrong governors change the 
eccentricity and the angle of advance of the eccentric. The 
center of the eccentric will lie somewhere along the path indicated 
in the figures, there being a definite position corresponding to 
every load on the engine. 

In the problems which, have been considered, a constant 
cut-off was assumed. This is the case only with an engine 
running at uniform load or equipped with a throttling governor. 
It therefore becomes necessary to investigate the method of ap- 
plying the valve diagrams to valve gears operated by a shaft 
governor. Since this governor controls the engine speed by chang- 
ing the point of cut-off, it is often called the '^ automatic cut-off 
governor." 



78 



VALVE GEARS 



With an outside admission valve and a fixed eccentric keyed 
to the shaft, when the crank is at OD (Fig. 86) the eccentric is 
at some position such as OE, 90° + 8 ahead of the crank, and 
these relative positions of the parts remain unchanged. 

With an eccentric not keyed to the shaft, but fastened to and 
controlled by a shaft governor, E is not fixed with relation to OD. 
When the governor adjusts itself to the speed the position of E is 
changed. Assume, for illustration, that the eccentric is pivoted 
to the flywheel at the point P (Fig. 86) and that the crank is held 
stationary at OD. By moving the governor from one extreme 





Fig. 86. 



Fig. 87. 



position to the other, the eccentric center may be made to 
occupy any position on the arc through E, drawn with P as a 
center. The engine may run for a time with the angle of ad- 
vance 8 and the eccentricity OE; then a change in the load 
may increase the speed, causing the governor to readjust itself 
and change the angle of advance to 5i and the eccentricity of 
OE,. 

On the Bilgram diagram this change in the position of the 
eccentric is shown by a change in the location of the lap circle 
center. Corresponding to the eccentricity OE the lap circle cen- 
ter is at Q (Fig. 87) ; a change in eccentricity to OEi (Fig. 86) 
shifts the lap circle center to Qi (Fig. 87). As all eccentric posi- 
tions for different positions of the governor must be on the arc of 
radius Y shown in Fig. 86, so must all lap circle centers be located 
on the similar arc of radius Y shown in Fig. 87. 

On the Zeuner diagram the change in the position of the 



SHAFT GOVERNORS 



79 



eccentric, due to a readjustment of the forces at the governor, is 
shown by changed valve circles. Corresponding to the eccen- 
tricity OE in Fig. 86 the valve circle is shown as a in Fig. 88. A 
change in the eccentricity to OEi demands a new valve circle, b, 
drawn with the new angle of advance and having a diameter 
equal to OEi. 




Fig. 88. 



CHAPTER VII 
VALVE SETTING 

There are two methods of setting valves : 

(a) By taking off the valve-chest cover and locating the valve 

by measurement. 

(b) By taking indicator. cards and adjusting the valve gear 

until the desired cards are obtained. 

The first step, in setting a valve by measurement, is to set the 
engine on center. Turn the engine until it is almost on center, 
make a mark on the cross-head and guide and, measuring with a 
tram from some fixed point, make a mark on the flywheel rim or 
the crank disk. Turn the engine in the same direction as before 
past the center until the marks on the cross-head and guide again 
coincide ; then, measuring with the tram from the same fixed point 
as before, make another mark on the flywheel. Bisect the dis- 
tance between the two marks on the wheel and bring the bisector 
opposite the tram. The engine will then be on center. It is 
important to keep the same brasses in contact with the cross-head 
and crank pins when the measurements are made with the tram, 
otherwise there will be an error due to lost motion. When the 
crank is near the dead center position, the eccentric stands in such 
a position that a slight movement of the shaft results in consider- 
able movement of the valve. For this reason the crank should 
be carefully placed before any measurements of the position of the 
valve are made. 

Having the engine on center, to set the valve with equal leads, 
adjust the angle between the crank and eccentric until the valve 
opens the port leading to the cylinder a slight amount. The 
width of the opening should be measured and recorded as a pre- 
liminary lead on that end — suppose, for example, that it is 1/8". 
Then the engine should be placed on the opposite dead center 
and the port opening on that end measured and recorded as the 
preliminary lead on that end — suppose it is 1/16". There is, 
then, 1/16" difference in the leads on the two ends. The valve 
must be moved on its stem, or the length of the valve rod changed, 
an amount equal to half the difference, or 1/32". This move- 

80 



VALVE SETTING 81 

ment of the valve should be in a direction away from the port 
having the smaller opening. 

By the method described, the leads on the two ends of the 
valve will be made equal, that is, the distance the valve uncovers 
the steam port will be the same for both dead-center positions. 
But while the leads are equal they are not necessarily the re- 
quired amount and it remains to set the eccentric to give the leads 
desired. Place the engine on center once more, and after loosen- 
ing the eccentric, move it around the shaft until the desired 
lead is obtained. As a final check, after securing the eccentric, 
the engine should be placed on the other dead-center and the 
lead proven correct. 

If the leads are to be different on the two ends, say 1/16" on 
the head end and 1/8" on the crank end, after setting the engine 
on center, set the valve and eccentric so that there is some port 
opening on the head end. Measure the opening and record it as a 
preliminary lead; suppose it is 1/32". Place the engine on the 
other dead center and measure the crank end lead; suppose it is 
1/4". The difference between these preliminary leads is 1/4" — 
1/32" = 7/32", while the difference between the desired leads is 
1/8" - 1/16" = 1/16". By shifting the valve 1/2 (7/32" - 
1/16") = 5/64" toward the crank end, the lead on the head end 
will become 1/32" -\- 5/64" = 7/64" and on the crank end 1/4" 
- 5/64" = 11/64". The difference between the leads will then 
be 11/64" - 7/64" = 1/16", which is the desired amount, but 
the leads are each too large by 3/64". They can be decreased to 
the required dimensions by reducing the angle of advance. 

In connection with Fig. 12 (p. 12), attention was called 
to the angularity of the eccentric rod which causes the valve 
to be drawn toward the shaft more than it would be if angularity 
were absent. In setting the valve to give certain desired leads, 
it is set slightly more toward the head end than it would be if 
the eccentric rod did not produce this effect. Thus, in setting 
the valve, the only serious distortion caused by the angular 
vibration of the eccentric rod is corrected. 

Setting the valve by the indicator consists simply in making 
the cards from the two ends as nearly alike as possible, by ad- 
justing the length of the valve rod, and then shifting the position 
of the eccentric until the events are properly timed. Of course 
the valve should be set for normal running conditions but cards 
should also be taken with the engine overloaded and under- 



82 



VALVE GEARS 




VALVE SETTING 83 

loaded to see that the valve acts properly throughout the range 
of running conditions. 

Fig. 89 shows a number of indicator cards which may be ob- 
tained with different valve conditions. A careful study of the 
causes of peculiar cards is the best guide to proper valve design 
and setting. 



CHAPTER VIII 
THE DESIGN OF SLIDE VALVES 

If at admission the crank is at the same angle from the dead 
center position on the two ends, that is, if the leads are equal 
as illustrated in Fig. 90, the steam laps must also be equal. 
This is readily seen when attention is called to the fact that the 
sum of the steam lap and lead must always equal the distance 
QM which in turn always equals Q'M'. 

With equal leads the percent of the stroke at which cut-off 
occurs will be different for the two ends. During the forward 

DW . D'W 

stroke it is jyi^r, and during the return ^fyfy^ and it is evident 

from the figure that DW is considerably more than D'W. The 
corresponding indicator cards are drawn to show that the dis- 
tribution of work between the two ends is quite different, the 
area of the head end card being greater than that of the crank end 
card. On account of the connecting rod crank method of 
transmitting motion it is impossible to have the leads and the 
cut-offs equal at the same time, without using some special form 
of gear. 

Unequal cards are undesirable; they should represent equal 
amounts of work so that the ratios of expansion will be equal 
and the turning effort on the crank shaft as nearly uniform as 
possible. To secure smoothness in running, the engineer 
endeavors to set the valve so that the indicator cards will b^ 
alike. 

The cut-offs can be made equal, as shown in Fig. 91, by 
making the leads and consequently the steam laps unequal. 
This results, however, in unequal admissions and a larger port 
opening on one end than on the other. 

Fig. 92 shows a curve known as the valve ellipse in which 
the ordinates represent the displacement of the valve from its 
mid-position and the abscissse the corresponding positions of 
the piston in its stroke. Positions to the right of mid-position 
are measured up from the line A-A and positions to the left are 
measured down. By laying off the amount of movement re- 

84 



THE DESIGN OF SLIDE VALVES 



85 




Fig. 90. — Equal leads and unequal cut-offs. 



86 



VALVE GEARS 




Fig. 91. — Unequal leads and equal cut-offs. 



THE DESIGN OF SLIDE VALVES 



87 



quired to draw the valve over the amount of the steam lap, the 
net port opening and the time when it occurs can be read from 
the figure. 

The cut-offs shown in Fig. 90 are represented at the points 
c and c' in Fig. 92 and the steam laps are shown as s and s'; c is 
a little later than 1/4 stroke and c' a little earlier than 1/4 stroke. 

The cut-offs shown in Fig. 91 are represented by the points 
d and d' in Fig. 92 and the steam laps by si and s'l; d and d' are 
both at 1/4 stroke. 




Stroke of Piston 

Fig. 92. — Valve ellipse. 



Thus it is seen from Figs. 90, 91 and 92 that the cut-offs 
can be equalized by increasing the head end and decreasing 
the crank end steam laps, but that this is accomplished at the 
expense of unequal leads and admissions. Fig. 92 further shows 
that the change of steam laps materially affects the size of the 
port openings and caused the throttling effect to be greater on 
the head end than on the crank end. For early cut-offs this 
will be more noticeable on the cards than for later cut-offs. 

Measurement of the cards in Fig. 90 shows the area of 
the head end card to be 112.5% of the area of the crank end 
card. In Fig. 91 the area of the head end card is only 92% 



88 VALVE GEARS 

of the area of the crank end card. In the first case the head 
end is doing more than half the work because the cut-off is later 
on that end. In the second case the cut-offs are the same but 
the head end is doing less than half the work because there 
is more wire-drawing and compression on that end. 

Evidently the best setting of the valve would give cards 
somewhere between those of Figs. 90 and 91, with a cut- 
off on the head end a little later than on the crank end but not 
so much later as is obtained with equal leads. 

It is not always possible or advisable to make the indicator 
cards iexactly equal in area, because to do this sometimes causes 
the engine to pound on account of the large difference in leads. 

The amount of compression should be practically the same 
on the two ends, its purpose being to cushion the reciprocating 
parts and fill the clearance space with steam at high pressure. 
Sometimes to secure quiet running of the engine it is necessary 
to give more compression on the head end than on the crank end 
because the inertia forces of the reciprocating parts are greater 
near the head end. 

Design of a D-Slide Valve. — A satisfactory method of de- 
signing slide valves is illustrated by the following example : 

Given.— An 8" X 12", 200 R.P.M. engine 
Cut-off at 25% of the stroke 
Compression at 70% of the stroke 
Lead 3/32". 

Design. — A D-slide valve for this engine. 
Let a = area of piston in square inches, 

V = average piston speed in feet per minute, = „ 

a' = area of maximum port opening in square inches, 
w = width of maximum port opening in inches, 
v' = nominal steam speed in feet per minute. 
Then 

a X V = a' X v' 

, axv 50-3X2x|x200 
a = 



V 9400 (see curve, Fig. 45) 

2.14 sq. in. 



THE DESIGN OF SLIDE VALVES 89 

The length of the port is usually about 3/4 the cylinder 
diameter; 

3/4X8 = 6" = length of port. 

2 14 
Therefore w = -^ = 0.357" 
o 

The various steps in the construction of the diagram to 
determine the laps, valve travel, and angle of advance, are 
explained in connection with Fig. 93. 

Instead of trying to make the admissions equal at the expense 
of unequal cut-offs, the attempt will be made to secure a better 
division of work between the two ends by making the leads 
unequal and in that way causing the cut-offs to be more nearly 
equal. In endeavoring to adjust a number of undesirable 
things without too much sacrifice of desirable characteristics, 
the designer must use his judgment. He will usually find it 
necessary to make more than one trial before satisfactory results 
are obtained. 

After drawing a pair of axes through 0, Fig. 93, and a circle 
to represent the path of the crank pin, locate m and m' at 25% 
of the stroke from each end. As a preliminary trial choose 
the leads so that the head end lead will be 1/16" and the crank 
end about 1/8", giving an average of 3/32" which agrees with 
the given data. It may be desirable to change these after 
partially completing the diagram. The head end lead of 1/16" 
should be laid off above the horizontal axis and the crank end 
lead 1/8" below the horizontal axis in the usual way. 

A short distance to the right of the point m choose the time 
of head end cut-off at g and swing the arc to locate the crank 
position OC. Just how much to the right of m this cut-off 
should be taken can not be told until after it is seen how the 
choice works out when compared with the other events. 

The computed width of maximum port opening is 0.357". 
If this is used on the head end the crank end opening will be 
unnecessarily large, yet if this value is used on the crank end 
the opening on the head end will not be sufficient and will 
cause too much throttling. As a compromise, a maximum port 
opening of 5/16" will first be tried on the head end. 

Having determined, temporarily at least, the crank position 
for head end cut-off, the head end lead, and the head end maxi- 
mum port opening, the diagram can be partially drawn to locate 



90 



VALVE GEARS 



the centers of the lap circles at Q and Q', and determine the valve 
travel. 




Fig. 93. 



With Q' as a center and tangent to the crank end lead line, 
the crank end steam lap can be drawn and the time of the crank 



THE DESIGN OF SLIDE VALVES 91 

end cut-off investigated. It should occur at m' or slightly earlier 
in the stroke. The crank end point of cut-off, (/, should not be 
more distant from m' than g is from m and preferably a little less. 

On account of the difference in leads and the fact that the 
reciprocating parts require more cushioning at the head end than 
at the crank end it is perfectly proper to have a little higher com- 
pression on the head end, the compression partly counteracting 
the smallness of the lead. 

The head end compression has been taken at 70% and this 
causes the release to be at 76% of the stroke. On the crank end 
the compression was first taken at 70 % but was later modified to 
73.5% because it was considered that this much additional area 
could be added to the crank end card and still have a pressure 
at the end of compression which would compare favorably with 
the pressure on the head end. 

The results shown by the valve diagram are : 

Head end Crank end 

Lead 1/16" 1/8" 

Steam lap 1.88" 1.82" 

Exhaust lap -0.02" +0.15" 

Valve travel 4 . 38" 

Angle of advance 62 . 5° 

Cut-off 27% 23.0% 

Release 76% 72.5% 

Compression 73.5% 70.0% 

Some engine builders neglect the effect of the angularity of the 
connecting rod in constructing the valve diagram and leave some 
extra stock on the valve so that the laps can be changed slightly 
after indicator cards have been taken with the engine on the 
testing floor. 

The width of the port in the valve seat is determined in the 

same way as the maximum port opening but using a nominal 

steam speed of 6000 ft. per minute for the machined openings in 

the seat and 5000 ft. per minute for the rough cored passages 

leading into the cylinder: 

Q400 
0.357" X gQQ^ = 0.56" say 9/16". 

9400 
0.357" X Iq^ = 0.67" say 11/16". 

Design of a Flat Double-ported Valve. — ^Let it be required to 
design the valve for a 10" X 10", 250 R.P.M. Ball engine; 
cut-off at 25% of the stroke, compression at 70% of the stroke, 



92 VALVE GEARS 

admission to occur when the crank is 4.5° before the dead center 
position. See Fig. 71. 

The maximum port opening is found in the same way as in 
the preceding problem: 

"^ X ^^ X 250 = a' X 9400 

a' = 3.48 sq. in. 
Assume the length of the port is 7". Then the width of the 
maximum port opening is 

^ = 0.497", say 0.5". 

The Ball valve is a double-ported flat valve and provides two 
passages for the inflow of steam. In order to provide a maximum 

0.5" 
port opening of 0.5^', the valve must be moved only -^ = 0.25" 

from the admission position. 

The complete diagram, shown in Fig. 94, is so similar to 
Fig. 93 that it needs no further explanation. 

In this type of engine the distance between the centers of 
the ports in the valves seat is usually about half the length of 
the cylinder bore. A 10" X 10" engine would have a piston 
about 4" wide and clearance distances of about 3/8". Then the 
length of the cylinder between the insides of the heads would be 

10" + 4" + 3/8" + 3/8" = 14f" 
and half this would be 7|". Knowing the dimensions of the 
steam and exhaust laps from the diagram and having decided 
upon the location of the cylinder ports the design of the valve can 
be completed as shown in Fig. 95. 

The necessary area for the passage of exhaust is determined in the 
same way as in the preceding problem, but the required width of 
the port at the valve seat is also affected by the thickness of the 
metal between the edge of the valve and the passage through the 
valve. When the valve is admitting steam this metal obstructs 
the cylinder port and the port must be made wide enough to give 
the required net opening. 

The width of the cylinder port at the valve seat to give the 

necessary passage area for the exhaust steam is 

9400 
0.497 X gggo = 0.78", 

but the width necessary for the inflow of steam is 

0.497" + 0.5" width of metal at the edge of the valve = 0.997". 



THE DESIGN OF SLIDE VALVES 



93 




Fig. 94. 



94 



VALVE GEARS 



The valve has been designed to give an opening 0.25'' wide past 
the outer edge and the passageway through the valve is 3/8" wide. 
This passageway is separated from the edge of the valve by a 
strip of metal 1/2" wide, so that the width of the port at the valve 
seat will be made 0.25" + 0.375" + 0.5"= 1.125 or 1^'. 



'H.E. 




^^/ - si" ■ -^$^''-^^\m 

Fig. 95. — Double ported flat valve for 10" X 10", 250 r.p.m. engine 

The width of the cylinder port below the valve seat is 

9400 
0.497 X ^^Q = 0.935" or say 1". 

Frequently the computed width of the passage in the valve is 
so small that it should be arbitrarily made larger to insure a 
smoother and better casting. In this problem, for example, 
the computed width was 1/4" and the valve could have been 
made with only 1/4" passage, but it was thought advisable to 
use 3/8". 

There is no satisfactory way to compute the width of metal 
at the ends of the valve, but since the valve rod connects with 



THE DESIGN OF SLIDE VALVES 



95 



one of these strips they must be made strong and rigid. Usually 
the thickness is fixed by the judgment of the designer and ribs 
are used to connect the strip and the body of the valve to obtain 
stiffness. 

This valve is usually designed so that its extreme position will 
be as shown in Fig. 96 a when the engine is running normally. 
The valve will generally be under the control of a shaft governor 




-b- 




FiG. 96. 



which will cause variable travel with changes in load. For a 
later cut-off due to a load above the normal the valve will move to 
some other extreme position as in Fig. 96 h, and partly or entirely 
close the opening through the valve itself. In some cases the 
valve may act single ported when wide open and double ported 
when partially opened. 

PROBLEM 14 

Construct a curve showing port openings as ordinates and corresponding 
piston positions as abscissae for the valve designed above: 

1. When the valve travel is as shown in Fig. 94. 

2. When the valve travel is increased to 4|", without any change in 
the valve itseK or in the cyhnder. 



96 



VALVE GEARS 



Investigation of the Action of a Shaft Governor. — If the 

engine is equipped with a shaft governor any movement of the 
governor affects the position of the eccentric center relative to the 
crank. The eccentric center is constrained to a definite path 
through which it can be moved by the governor and for every 
position of the eccentric center on this path there is a correspond- 
ing lap circle center on the Bilgram diagram. The locus of the 




Fig. 97. — Governor and eccentric pin, Ball and Wood engine. 

lap circle centers, or the points Q, should be shown on the diagram. 
In a problem of design the path chosen depends on the governor 
to be used. In a problem of investigation of an engine already 
built, the path can be determined by measuring the mechanism 
on the engine. 

The governor of a Ball and Wood engine is shown in Fig. 97. 
The governor -arms are attached to a plate C which is set eccen- 
tric with the shaft and the eccentric pin, E, is in. turn eccentric 
with the plate. 



THE DESIGN OF SLIDE VALVES 



97 



To determine the locus of E for all positions of the governor, 
proceed as follows : 

1. Place the engine accurately on center. 

2. Locate the center of the plate C relative to the shaft and 
crank. 

3. Locate the pin E with respect to the center of the 
plate C. 

Fig. 98 shows the measurements taken on a 10" X 12" Ball and 
Wood engine in the laboratory at the University of Michigan. 
With the crank held at OD' and with the spring disconnected 
the governor could be moved through its entire range and the 
eccentric center caused to assume any position on the path E. 





Fig. 98. 



Fig. 99. 



Plumb lines, a protractor and calipers were used in locating the 
point C and the extreme positions of the center of the eccentric pin 
E. Knowing the possible positions of the eccentric center, the 
locus Q of the corresponding lap circle centers can be constructed 
as shown in Fig. 99, and then the valve diagram drawn, keep- 
ing in mind that the lap circle center must be on the path Q, 

For further illustration consider the engine shown in Fig. 100. 
To construct a complete valve diagram for the engine, first set 
the valve properly and locate the pivot pin P (Fig. 101) about 
which the eccentric is swung by the governor. After measur- 
ing the steam lap, exhaust lap and lead the engine should 
be turned over by hand and the valve travel measured. 

7 



98 



VALVE GEARS 



While turning the engine by hand the governor is in an extreme 
position causing maximum valve travel and latest cut-off. Know- 
ing the laps, lead and the maximum valve travel, construct the 




Fig. 100. 

full lines of Fig. 102, thus determining the angle of advance 
for latest cut-off. This gives sufficient information to locate 
one position of the eccentric center, such as E, in Fig. 101. An 
arc through E with center at P gives the path through which the 
governor can move the eccentric. The corresponding path of 




Fig. 101. 

lap circle centers, Q, should be shown on the diagram in Fig. 102. 
It is a simple matter, after disconnecting the springs, to block 
the governor in the extreme position and measure the minimum 



THE DESIGN OF SLIDE VALVES 



99 



valve travel. This determines the limiting positions of the 
eccentric and the length of the arc E in Fig. 101 and Q in 
Fig. 102. 

In designing a valve to be controlled by a shaft governor, 
as yet undesigned, the eccentric center is first determined for the 
normal running of the engine. The path through which the 
governor will move the eccentric may then be chosen. In mak- 
ing this choice it must be remembered that it is not desirable 




Fig. 102. 



to have the time of admission vary much with the different 
cut-offs. Also such a path must be chosen that the governor 
will be well proportioned to the flywheel of the engine. Inspec- 
tion of engines in successful operation will give the best ideas 
about these proportions. 



CHAPTER IX 

VALVES WITH RIDING CUT-OFF 

When a single slide valve is used on an engine to regulate all 
the events, if admission occurs slightly before dead center and 




Fig. 103. 

cut-off early in the stroke, the angle of advance determined by 
these two events is large, as shown diagrammatically in Fig. 103. 

100 



VALVES WITH RIDING CUT-OFF 



101 



This large angle of advance causes either release or compression, 
or both, to be early. If one is improved it must be at the expense 
of the other. In Fig. 103 the crank position for release has 
been assumed at 85% of the stroke. Compression is thus 
fixed at 55% of the stroke. The probable card is drawn below 
the diagram and additional dotted lines inserted to show the 
effect of trying to make the compression occur later. 

On high speed engines a rather high compression pressure is 
desirable to absorb the inertia force of the reciprocating parts at 
the end of the stroke; but on lower speed engines so much com- 
pression is not -necessary, and is undesirable because it reduces 
the area of the card and the work of the engine per stroke. 

One means of surmounting this difficulty is to use what is 
called a riding cut-off valve. Three representative valves of 
this class will be discussed — the Meyer, the Buckeye, and the 
Mclntosh-Seymour. 



Index Plate 




Handwheel 

Mam Valve 
Cut-off Valve 



Fig. 104. — Meyer valve. 

The Meyer valye (Fig. 104) is an example of a valve with 
an independent and adjustable cut-off. This type of valve is 
frequently used on the steam cylinders of air compressors and 
pumps. It is composed of two parts — the main valve regu- 
lating admission, release, and compression, and the cut-off 
valve regulating only the cut-off. The main valve acts the 
same as a D-slide valve, the only difference being that a port P is 



102 VALVE GEARS 

formed in the valve itself and steam enters through P instead of 
past the outer edge. The main valve is operated by a fixed eccen- 
tric and does not cut-off until late in the stroke. The cut-off valve 
consists of two blocks which slide back and forth on the back of 
the main valve. The valve rod operating the cut-off valve (Fig. 
104) is extended .through the rear of the steam chest and the 
end squared to fit a stationary handwheel in which it slides as 
the engine runs. The rod is threaded for a portion of its length 
with two separate sets of threads, one set being right-handed 
and the other left-handed. One of the cut-off blocks is on the 
right-handed and the other on the left-handed thread. Turning 
the wheel separates the blocks or draws them together thus 
making the cut-off earlier or later as may be desired. This 
can be done while the engine is running without affecting the 
timing of the other events. An index plate mounted on the 
wheel bracket indicates the percent of the stroke at which cut-off 
is occurring. 

The valves are operated independently by separate eccentrics 
and are designed so that at admission the cut-off valve is not 
obstructing the port P; at cut-off the block slips over the port P 
while P is yet in communication with the cylinder. 

A series of views of the Meyer valve in various positions is 
given in Figs. 105 to 111 with a common vertical reference line 
from which the displacement can be measured. Such valves 
can be investigated or designed by means of either the Zeuner 
or Bilgram diagram by placing the diagram for one valve on 
top of the diagram for the other valve. If the travel, steam lap, 
exhaust lap, and lead of the main valve and the eccentricity 
and angle of advance of the cut-off eccentric are given, Fig. 112 
can be constructed with Q and Q" as the lap circle centers for 
the main valve and the cut-off valve respectively. 

Fig. 105 shows both valves in mid-position with the lap dimen- 
sions indicated. The valves must be put in this position to 
measure the laps but when properly set for operation both 
valves will not be in mid-position at the same time. Fig, 112 
shows the position of the crank, ON, when the main valve is in 
mid-position; a perpendicular from Q" to NO (produced) gives 
the displacement {Q"n") of the cut-off valve from mid-position. 
The valves are shown in their correct relative positions, for crank 
at ON, in Fig. 106. 

Referring again to the diagram (Fig. 112) it is seen that for 



VALVES WITH RIDING CUT-OFF 



103 



Fig. 112. 




( Scale of Diagrams 

twice that of Sketches.) 



>I^K 



Fig. 10," 



r ,.Mai'n Valve >\^k 




w/////mm 



^ 




mv////^ 



e' 5', 
Both Valves in Mid- Position. 



Fig. 106 




Main Valve in Mid-Position ; Crank atON. 



Fig. 107 




Admission, 






W 



w 







"^ 



Cut- off J Crank at OC 




Fig. 109. ^ 



Fig. 110. 



i 



Cut-off J Crank at OC^., 



''^//////////^ 



;4-'ci'o>i 




v/rw/M///' 



Cut-off-, Crank at OD. 



Fig. Ill 




"'W////>- 



!#• 



v/r w/////Ay- 



Max. Relative Displacement of Valves 
PrevioustoCut-offby MainValve. 



Figs. 105 to 114. — Various positions of the Meyer valve. 



104 VALVE GEARS 

the admission position, OA, the main valve is the distance Qa 
from mid-position and the cut-off valve Q'^a" to the same side 
of mid-position. In Fig. 107 the valves are shown in their cor- 
rect relative positions for crank at OA. 

As the crank continues its rotation in a clockwise direction the 
main valve continues to move to the right and increase the 
opening to the cylinder port. Soon after the crank position OA 
is passed the cut-off valve changes its direction of motion and 
starts toward the left, and when crank position OC (Fig. 113) is 
reached it has covered the port in the main valve as shown in 
Fig. 108. The diagram (Fig. 113) shows that the main valve is 
the distance Qc from mid-position and the cut-oif valve the 
distance Q"c". Imagine the valves shown in Fig. 108 to be 
slipped back to mid-position. The cut-off valve would fail to 
cover the port P by the amount Qc~Q"c", i.e., it would have a 
negative steam lap of that amount. 

Looking at this from another standpoint, suppose cut-off 
is desired when the crank is at OC (Fig. 113) what steam lap must 
the cut-off blocks have? When the crank is at OC the main valve 
is a distance Qc and the cut-off valve a distance Q"c" to the right 
of mid-position. The main valve has traveled farther than the 
cut-off valve and, to have cut-off occur at this time, the cut-off 
valve must have a negative steam lap equal to the difference 
between Q"c" and Qc. This amount can be easily determined 
and represented on the diagram by drawing QV parallel to OC 
and then a circle about Q" tangent to QV . 

By drawing the blocks close together the cut-off can be delayed 
until the crank reaches the position OCi, (Fig. 113) at which time 
the main valve closes the port to the cylinder. Fig. 113 shows that 
the main valve is then Qh to one side of mid-position and the 
cut-off valve Q "h " to the other side of mid-position. The valves 
are shown in the act of cutting-off in Fig. 109. 

By separating the blocks the cut-off can be made earlier. To 
have cut-off at zero stroke, or at crank position OD (Fig. 114), the 
event must occur when the main valve is the distance of Qd from 
mid-position and the cut-off valve Q"0 from mid-position. 
If the valves as shown in Fig. 110 are imagined slid back to mid- 
position the steam lap of the cut-off block is found to be 
Q"0~Qd, i.e., equal to the radius of the circle drawn about Q " 
(Fig. 114) tangent to a line Qr parallel to the crank position at 
cut-off. As the crank revolves from OD (Fig. 114) the main valve 



VALVES WITH RIDING CUT-OFF 



105 



continues to move to the right for a time then reverses and moves 
to the left, finally reaching the position shown in Fig. 1 1 1 , Q6 to the 
right of mid-position, when the crank is at OCi,. During this 
movement of the main valve the cut-off valve moves con- 
tinuously to the left, and when the crank is at OCi, the cut-off 
valve is the distance Q "h " to the left of mid-position. The total 
maximum relative displacement of the two valves during the 
rotation of the crank from OD to OC^ is Qh + Q"h'\ 

To prevent steam entering the cylinder past the back edge of the 
cut-off valve after it has cut-off, the block must be made wide 




Fig. 115. 



enough to lap the port in the main valve until the main valve cuts 
off the passage P from the cylinder. That is, the width of the 
block should be Q"r + Q"h" + Qh + width of port P + an 
allowance of about 1/8". 

The necessary steam lap of the cut-off blocks to bring cut- 
off at any desired time can be determined as illustrated in 
Fig. 115. If cut-off is to be at the crank position OC, draw 



106 



VALVE GEARS 



QV parallel to OC. The dotted circle about Q'^ and tan- 
gent to QV shows the necessary negative steam lap. Simi- 
larly for cut-off at any crank position, such as OM, draw Qt 
parallel to OM, and a circle tangent to this shows the required 
positive lap. For cut-off at crank position ON, parallel to QQ", 
zero steam lap is required; for earlier cut-off the steam lap becomes 
a larger positive amount and for later cut-off a larger negative 
amount. The largest negative lap required is Q"h" + Qh, for 
that will give cut-off at crank position OCi^, the same time at 
which the main valve cuts-off. 




In designing a Meyer Valve readmission can be guarded against 
by properly locating Q" and making the blocks of the correct 
width. In Fig. 116, which is a Bilgram diagram for the main 
valve, QX is drawn perpendicular to OC^. The lap circle center 
Q" for the cut-off valve may be located arbitrarily but it should 
not be below QX. On engines which are to run in either direc- 
tion, as hoisting and marine engines, the angle of advance, 81, 
of the cut-off eccentric should be 90°, so that the eccentric is in 
position to run in either direction. For such an engine Q" would 
be on the vertical axis through 0. On engines which are not to 
be reversed it is customary to make the angle of advance of the 



VALVES WITH RIDING CUT-OFF 



107 



cut-off eccentric less than 90° because this allows narrower cut-off 
blocks and less movement to change the cut-off. 

If the blocks are drawn close together to obtain a late cut-off 
they might cut-off and instantly open again, giving readmission. 
The throw of the cut-off valve relative to the main valve is Q"Q; 
therefore if a negative lap Q'^Q is given to the cut-off valve it 
will close the port in the main valve and immediately open it 
again when the crank is at OF, perpendicular to Q"Q. This is 
after the main valve has cut-off so it does not affect the steam 
distribution. If a greater negative lap is given to the cut-off 
valve it will never close the port P. 

If Q" is located on the line QX (QX is perpendicular to OCl) 
and the cut-off valve given a negative lap of Q"Q it will close 




Fig. 117. 



Fig. 118. 



the port in the main valve and immediately open it again when 
the crank is at 00^; this will occur simultaneously with the 
cut-off of the main valve and the steam distribution will still be 
unaffected by the cut-off valve. 

If Q" is located below the line QX, as shown in Fig. 117, the 
cut-oft^ valve will cut-off and immediately admit again when the 
crank is at OH. This is before the main valve has closed the 
port into the cylinder and the steam distribution will therefore 
be affected. 

The motion of the cut-off valve may be considered relative 
to the main valve (Fig. 118) and the diagram for the cut-off 
valve constructed as for any ordinary slide valve on a stationary 
seat. Q then becomes the center of the diagram. The throw 
of the cut-off valve relative to the main valve, Q"Q, is the radius 



108 



VALVE GEARS 




• VALVES WITH RIDING CUT-OFF 109 

of the eccentric circle. For cut-off at crank position QM the 
steam lap Q"M is required. The cut-off valve uncovers the port 
in the main valve when the crank is at QA". 

Summary. — 1. Design the main valve just like any slide valve 
with a j&xed cut-off, making the cut-off the latest ever desired 
(Fig. 116). 

2. Locate Q" (Fig. 116) on or above the line QX which is drawn 
perpendicular to the crank position for latest cut-off. 

3. Make the width of cut-off blocks sufficient to insure against 
readmission. 

Buckeye Engine Valve Gear. — A very interesting riding cut- 
off valve used on the Buckeye engine, manufactured in Salem, 
Ohio, is shown in Figs. 119 and 120. Both valves are of the 
piston type and the cut-off valve slides within the main valve. 
The main valve, which regulates all the events except cut-off, 
admits steam through a port in the valve itself and exhausts 
past the outer edge. Cut-off is determined by the inner or cut- 
off valve. 

Each valve has its own eccentric, that of the main valve 
being keyed to the shaft, while that of the cut-off valve is 
connected to a shaft governor which changes the angle of advance 
but not the eccentricity. 

The valve rods are connected to the eccentric rods through 
a compound lever, or rocker, which transmits the motion from 
the eccentric rods in such a way that the travel of the cut-off 
valve relative to the main valve is constant, irrespective of the 
time of cut-off. This constant travel of the cut-off valve, on 
the main valve, is a desirable feature since it eliminates the 
possibility of wearing shoulders which would cause leakage with 
variable travel. 

Fig. 121 shows the compound rocker and Fig. 122 a line dia- 
gram of the gear, drawn in a conventional form. The main 
valve rod and the main eccentric rod are attached to the upper 
end of the rocker at Vm and Em respectively. The lower end of 
the rocker E^BA is pivoted to the engine frame at A. At Vc 
the valve rod of the cut-off valve is attached to the rocker DBEc, 
the pivot B being in the rocker EmBA. The cut-off eccentric 
rod is attached to Ec. B is at or near the center of the rocker. 

Since the main valve takes steam inside, the position of the 
valve and eccentric for dead center is as shown in Fig. 123. 
The horizontal motion of the eccentric is transmitted to the 



110 



VALVE GEARS 




VALVES WITH RIDING CUT-OFF 



111 



valve without change and the main valve is similar to an ordinary 
valve with inside admission; it is not affected by the rocker. 

Turning now to the cut-off eccentric and rocker, Fig. 122, as- 
sume that DB = BEc. Ec swings a constant amount, equal to the 
cut-off eccentric radius, to each side of the stationary point A. 
Since only the angle of advance of the cut-off eccentric is changed 




Ec ■ 



Fig. 121. 




the magnitude of the movement of Ec is constant. If EmA is held 
stationary, D swings the same amount as Ec since DB = BEc. 
If EmA is not held stationary but is moved by the eccentric 
ei, the pivot point B is carried with the rocker so that the move- 
ment of D relative to E^. is the same as before. The time in 
the stroke when D and Em pass each other will vary with the 
time of cut-off. 




Mclntosh-Seymour Engine Valve Gear. — The valves of the 
Mclntosh-Seymour Engine, built in Auburn, N. Y., belong to 
the type known as gridiron valves. A riding cut-off is supplied 
on the steam valves and the arrangement is as shown in Fig. 
124. There is a separate valve for admission and for exhaust 
at each end of the cylinder, thus making the admission events 
independent of the exhaust events and all the events on one 
end independent of those on the other. 

The motion of the main valves is derived from a lay-shaft 
called the main rock shaft which is rocked by a fixed eccentric, 
and this rocking motion, considerably distorted, is transmitted 
to the valve by a crank and toggle. The distortions are such 
as to cause the valve to move quickly in opening, pause when 
full open, and remain almost stationary when closed. On 
account of the many ports and the pause when full open, a small 



112 



VALVE GEARS 




VALVES WITH HIDING CUT-OFF 113 

movement of the valve admits ample steam. This movement 
ranges from about 1/2" in the smaller engines to 1 1/2" in the 
largest sizes. 

The exhaust valves are driven by direct connection to a crank 
on the same lay-shaft which operates the main steam valves. 

Another lay shaft called the cut-off rock shaft, rocked by 
an eccentric controlled by a shaft governor, operates the cut-off 
valves through a crank, long link, and bell crank lever. For 
ordinary loads the main valves and cut-off valves move in opposite 
directions, when cut-off occurs, thus giving quick closure. 

The gear, being positive in action, is not limited to slow speeds 
of operation and is analogous to the four-valve engines using 
Corliss valves without trip motion. 

In designing such a gear, where the motion of the eccentric 
is not transmitted direct to the valve, but is distorted by inter- 
posed levers and cranks, cut and try methods must be used. 

A layout showing a tentative arrangement of levers and rods 
which will transmit the motion should first be made, and then 
the whole combination traced through a complete revolution 
to determine the action of each individual part, and particularly 
that of the valve. It is not to be expected that the first trial 
will give the best result but it will indicate where changes should 
be made to approach closer to the desired result. The Zeuner 
diagram applied to such gears will not be made up of valve circles 
but irregular curves. 



CHAPTER X 



PUMP VALVES 



As a direct acting pump has no revolving shaft from which 
to take its valve motion, the valve gear must be of some different 
form from that employed with the steam engine. The valve 
proper is similar to the engine valve and is usually flat and single 
ported. It is moved either by steam, as in the case of the Cam- 
eron pump, by mechanical means, as in the Worthington duplex 
pump, or by a combination of steam and mechanical means, as in 
the Blake-Knowles. 

Pump valves must be made positive in action, that is to say, 
the operation at the slowest speed must be perfectly continuous 
and the pump never liable to stap as the valve passes its central 
position. Furthermore, the pump should always start regardless 
of the position in which it was previously stopped. 




Fig. 125. — Steam cylinder of Cameron pump. 

The Cameron pump is a good example of the class whose 
valve is operated by steam acting on an auxiliary plunger con- 
nected to the valve. A section through the steam cylinder 

111 



PVMP VALVES 115 

of a Cameron pump is shown in Fig. 125. The plunger F 
moves the valve G which admits steam to the cylinder A. The 
plunger F is cylindrical, with a cavity at each end filled 
with steam from the steam chest L. A small hole in each end 
of F allows steam to pass out and fill the space at the end of F. 
In the position shown the pressure on both ends of F is the same 
so that F is balanced. A projection, G, on the back of the valve, 
fits into a slot in F so that as F moves it carries G with it. 

As the piston C nears the end of its stroke to the left, it comes 
in contact with the reverse valve, J, moves it to the left and thus 
connects passage E with the main exhaust. This unbalances the 
plunger F and it moves to the left and closes port E. The 
valve is carried with the plunger and allows steam to be admitted 
to the left of piston C. The piston then moves to the right thereby 
releasing the valve I which is immediately closed by steam entering 
through the passage K. 

The pump necessarily takes steam during the full stroke since 
the resistance is constant and there is no flywheel to store up 
energy. The form of the ports in the cylinder causes enough 
steam to be entrapped to cushion the piston at the end of the 
stroke. 

Worthington Duplex Pump. — In 1859 Henry R. Worthington 
invented the Duplex Pump. A unit consists of two separate 
pumps of the same dimensions, side by side on a common frame. 
The piston rod of one pump operates the valve rod of the other. 
Fig. 126 shows the general appearance of the pump and Fig. 127 
a section through one side. The action is as follows : suppose the 
piston in the foreground of Fig. 126 moves to the right, it causes 
A to move to the right, and A, working through a reverse lever, 
draws the valve of the companion pump to the left. This admits 
steam to the right of the other piston and it moves to the left, 
carrying with it an arm similar to A which working through 
a lever draws the valve of the first pump to the left. This 
admits steam to the right of the first piston and sends it to the 
left. 

There is a slight pause at the end of each stroke while one pis- 
ton waits for its valve to be moved by the other. This pause is 
claimed to be an advantage since it allows the water valves to 
seat properly and noiselessly. 

The section through the cylinder (Fig. 127) shows two ports 
at each end of the cylinder. The outer one is for live steam and the 



116 



VALVE GEARS 



inner one for exhaust. The valve has neither lap nor lead on 
either the steam or exhaust side. Toward the end of the stroke 



fit 




FiG. 126. — Worthington duplex pump. 




Fig. 127.— Longitudinal section of Worthington pump. 

the piston covers the exhaust port and entraps some steam be- 
tween it and the cylinder head, the steam port being covered by 



PUMP VALVES 



117 




Fig. 128. — Cushion valve. 



This permits the piston 



the valve. This entrapped steam cushions the piston and brings 
it to rest quietly. 

On some of the larger pumps (14" or more in diameter) it has 
been found advantageous to 
regulate the amount of cushion- 



ing by placing a valve between 
the steam and exhaust ports. 
This valve, called a dash 
relief or cushion valve is shown 
in Fig. 128. It is so adjusted 
as to allow some of the cushion 
steam to leak out into the exhaust. 
to take a somewhat longer stroke. 

The Blake-Knowles valve gear (Figs. 129 and 130) consists 
of a main valve 1, an auxiliary valve 2, working on the back of 
the main valve, and an auxiliary plunger 3, working in an aux- 
iliary cylinder 4, which is bolted to the main cylinder 5. 

The main valve 1 is controlled by the movements of auxiliary 
plunger 3. The auxiliary valve 2 is actuated by valve rod 6 and 
valve rod link 7, which are moved by lever 8, tappet 9, and ad- 
justable collars 10 and 11. The auxiliary valve travel is at right 
angles to that of the main valve. 

The main cylinder 5 is so designed that the steam ports for 
each end are arranged on opposite sides of the center line, hence, 
in the sectional view only the ports for one end fully appear. 
Of these 12, 13 and 14 are the main steam ports; 15 is the starting 
port and 17 and 18 the auxiliary ports. The small drilled hole 
19 connects the main and starting ports and serves as a cushion 
relief. The main valve is provided with auxiliary ports, 21, 22 
and 36, and the auxiliary cylinder with one port at each end, of 
which 17 (Fig. 129), leading to right-hand end, shows a partial 
view, and 18 (Fig. 130), leading to left-hand end, shows a full 
view. 

The main steam inlet is at 25 and entering steam passes up 
through passage 26 into chamber 27 in the auxiliary cylinder. 
The exhaust outlet is at 28. 

Assume main valve 1 and auxiliary plunger 3 to be at the right, 
as shown (Fig. 130), being held in this position by steam, which is 
admitted by ports 21 and 18 to the left of auxiliary plunger 3. 
With the main valve in this position steam will pass through the 
starting port, corresponding to 15 but at the opposite end of the 



118 



VALVE GEARS 




o 



PQ 

O 

ill 



C5 

t-H 

fin 



PUMP VALVES 119 

cylinder, and main port 14, to the right of main piston 29, Fig. 
129, causing it to move to the left, steam exhausting at the same 
time from the opposite side of main piston, through port 12 to 
cavity 30 in main valve 1, and out to exhaust opening 28 through 
port 13 in main cylinder 5. 

As the main piston 29 nears the end of its stroke, lever 8, 
carrying with it tappet 9, and actuated by cross-head 31 on main 
piston rod 32, engages with collar 10 and moves valve rod link 7, 
valve rod 6, and auxiliary valve 2, to the left until auxiliary port 
22 is opened to admit steam to the right of auxiliary plunger 3, 
while auxiliary port 21 is opened for exhaust. 

Steam is now admitted at the right of auxiliary plunger 3, 
through ports 22 and 17, while at the same time steam is exhausted 
from the left of auxiliary plunger 3, through ports 18, 21, cavity 
33 and port 36. The auxiliary plunger 3 and main valve 1 are 
moved to the left until port 21 is closed to exhaust by the edge 
of cavity 33 in auxiliary valve 2. Port 21 being closed, auxiliary 
plunger 3 cushions on the steam confined between its end and 
auxiliary cylinder head 34, coming to rest with main valve 1 in 
proper position for the return stroke of main piston. 

The main port 12 enters the cylinder at some distance from 
the end, and after main piston 29 moves over and closes the port, 
the only outlet for the exhaust steam is through starting port 15 
and cushion relief hole 19. Owing to this restriction of the ex- 
haust outlet the speed of the main piston is reduced as it ap- 
proaches the end of its stroke, before steam is admitted for the 
return stroke, thus permitting the pump valves to seat quietly 
and the movement of the piston to be reversed without shock. 



CHAPTER XI 

REVERSING GEARS 

Engines which must be reversed in direction of rotation, 
frequently, on account of their particular application must be 
equipped with some form of valve gear which will permit this 
being done easily by the operator. Among the most common 
gears of this kind may be mentioned: 

1. The Stephenson Link Motion. 

2. The Walschaert Valve Gear. 

3. The Joy Valve Gear. 

Besides providing a means of reversing the engines at will, 
these gears also permit the cut-off to be varied by the operator 
while the engine is running. 

The Stephenson Link Motion. — In past years the Stephenson 
link motion (invented in 1843) received more attention in this 
country than the other gears mentioned and was extensively used 
on hoisting engines, marine engines, and especially on locomotives. 
Until about 1904, practically all the locomotives in the United 
States were equipped with this gear. 

Possibly the idea of the Stephenson link motion grew out of the 
fact that an engine can be reversed by shifting the eccentric from 

position Od (Fig. 131) to position 
Od'. If a disk with a slot dd' is 
keyed to the end of the shaft and 
^,, i „, the end of the eccentric rod is fitted 
i with a block which can be slid along 
the slot dd' and secured at any de- 
^ sired point, the angular position 
■pjQ 23-1^ ' and the length of the eccentric arm 

can be changed at will. Thus the 
eccentricity might be Od, Od", or Od'. When the eccentric 
radius is in the neighborhood of Od" the angle of advance is 
greater and the eccentricity less than Od and an earlier cut-off 
is obtained; a shift past the point d'" would reverse the engine. 
The practical mechanism, incorporating the general ideas 
expressed above, is shown diagrammatically in Fig. 132. It 

120 




REVERSING GEARS 



121 



will be noted that the link between the two eccentrics has 
been moved away from the shaft, 0, and that eccentric rods 
have been inserted. The link can be shifted by means of a 
bell crank lever so that d or d' or any intermediate point along 
the link will stand opposite the block h in the end of the valve rod. 
The arm by which the link is shifted is at one side of the link, 
thus allowing the latter to swing in the plane of motion of the 
valve. If the link is shifted downward so that d is opposite the 
block, the valve will receive motion from eccentric Oe alone; 
if the link is shifted to the other extreme position so that d' 
is opposite the block the valve will receive motion from eccentric 




Fig. 132. — Stephenson link mechanism. 



Oe' alone; but for any position of the link between these two 
extremes the valve will receive motion from a virtual or equiva- 
lent eccentric whose angular position is somewhere between Oe 
and Oe', and whose radius is something less than Oe. 

Locomotives have two engines with cranks at right angles, 
and each valve gear is provided with a link. Both links are 
shifted simultaneously by means of a lever in the cab which 
rotates the shaft m. 

When the link is in such a position that d (Fig. 132) is in 
line with h it is said to be in fvll gear ahead; when in such 
a position that d' is in line with h it is in fidl gear hack. When 
6 is -half way between d and d' the link is in mid gear. Shift- 
ing the link from the full gear toward the mid-gear position 
is termed notching up or hooking up. Positions interme- 
diate between mid -gear and full gear are expressed in fractions 
of the shift from mid to full gear. The mechanism is made 



122 VALVE GEARS 

so that the lever on the notched arc in the cab is relatively in 
the same position as the block in the link. 

When starting the engine the link should first be thrown into 
the full-gear position so that a late cut-off will be obtained; then 
with the throttle partly open a powerful, steady force will be 
exerted on the drivers. As the inertia is gradually overcome 
and the engine increases in speed the link should be hooked 
up to give an earlier cut-off and better steam distribution. 

It is necessary to distinguish between two possible arrange- 
ments of the eccentric rods. . If the rods stand clear of each 
other when the eccentrics are between the link and the 



Fig. 133. — Open rods and crossed rods and their effect on lead. 

vertical through 0, as shown in Figs. 132 and 133(A), they are 
called open rods. If they stand as shown in Fig. 133 (-B) 
they are called crossed rods. The most important difference 
in these two arrangements of rods is the effect on the lead when 
the link is shifted. With open rods, shifting the link from 
mid-gear to full-gear position decreases the lead as shown in 
Fig. 133(A). Fig. 133(5) shows the opposite effect for crossed 
rods. The amount of variation of lead from mid to full gear 
depends on the radius of curvature of the link and is increased 
as the radius is decreased. 

Approximate Layout. — A convenient method of obtaining the 
approximate solution of link motion problems is by means of 
what is known as an equivalent or virtual eccentric. An equiva- 
lent eccentric is an imaginary eccentric which, if used, would give 




REVERSING GEARS 123 

nearly the same movement to the valve as the more complicated 
gear. The following method of finding the equivalent eccentric 
is often used for open rods; a similar method can be used for 
crossed rods, but as crossed rods are seldom used that construc- 
tion will not be discussed here. 

The point of cut-off when in full gear should be chosen at about 
85% of the stroke and the valve allowed to overtravel the port 
about one-fourth of the width of the port. A lead of 1/16" may be 
assumed. Construct a Bilgram diagram to determine the 
length of the equivalent 
eccentric and its angle of 
advance for latest cut-off. 

The general proportions 
of the engine on which the "~ 
link motion is placed usu- 
ally determine the length 
of the eccentric rods. The ' ^' 

distance from the center of F^^- 134.--Graphical determination of 

equivalent eccentric lor open rods, 
the eccentric to the center 

of the link arc, measured along the eccentric rod, is usually taken 

for the radius of the center of the link arc. 

Having decided upon the general proportions of the gear, a 
skeleton diagram can be laid out as shown in Fig. 134, with the 
equivalent eccentric for full gear represented by Od and Od'. 
On Od as a diameter construct a circle and by drawing Oa 
and dgm through the intersection point, g, locate the point m 
on the axis line. Through d, m, and d' draw a circular arc; this 
will give the locus of all equivalent eccentrics for various link 
positions. When the link is in half gear it will give a motion 
to the valve which is approximately the same as the motion 
which it would receive from a single eccentric Od", where d" 
is half way between d and m. Od" represents the equivalent 
eccentric, both in radius and angular position. When the link 
is in mid-gear the motion of the valve will be approximately 
the same as it would receive from an equivalent eccentric Om. 

The equivalent eccentric being known, the motion of the 
valve may be analyzed for any assumed position of the link as in 
the case of a simple gear, but it must be remembered that this 
method gives results which are only approximately correct. 

The Walschaert Valve Gear. — The Walschaert valve gear be- 
longs to the general class known as radial gears. In this class 



124 



VALVE GEARS 



are included a number of mechanisms which give practically 
harmonic motion to the valve and enable the steam distribution 
and the direction of rotation to be altered. 

A Swiss named Egide Walschaerts invented the Walschaert gear 
in 1844 so that it is about as old as the Stephenson link motion. 
For many years it has been used on locomotives in continental 
Europe but was almost unknown in this country until after 1900. 
The American Locomotive Company is now equipping nearly all 
of its locomotives with the Walschaert gear for the following 
reasons: 

In modern locomotives the use of an outside valve gear is 
practically necessary, because with the weight and power of the 




Fig. 135. — Walschaert valve gear. 

locomotives of today it is almost impossible to get a satisfactory 
design of Stephenson link motion between the frames. The 
parts are necessarily so large that there is hardly room for them. 
With the Walschaert valve gear, which is outside of the frames, 
this difficulty is overcome. 

In the modern locomotive the eccentrics and straps of the 
Stephenson link motion are necessarily very heavy and wide. 
The rubbing speed is consequently high and results in rapid wear. 
This, in combination with the rockers and transmission bars, 
permits the accumulation of a great amount of lost motion in the 



REVERSING GEARS 



125 



gear. With the Walschaert valve gear there are no large eccen- 
trics, only hardened pins and bushings. As a result it is much 
more easily main- 
tained than the 
Stephenson motion. 

Being located out- 
side of the frames, 
the Walschaert gear 
is easily accessible 
for proper lubrica- 
tion and attention 
by the engineer. 

By removing the 
valve gear from be- 
tween the frames a 
better opportunity 
is afforded to intro- 
duce stronger frame 
bracing, thus reduc- 
ing the possibility of 
frame failure. 

Fig. 135 shows 
the arrangement of 
the parts which 
make up the Wals- 
chaert gear. Two 
positions of the 
mechanism are 
shown, A for admit- 
ting steam at the 
forward end, and B 
for admitting steam 
at the back end. 
Fig. 136 is a skeleton 
diagram of the gear. 
The link is trun- 
nioned at its middle 
point and rocked by 
means of an eccentric 
rod whose motion is derived from an eccentric crank secured to 
the main crank pin. The movement of the link is transmitted 







126 VALVE GEARS 

to the valve stem by a radius rod whose length is equal to the 
radius of curvature of the link. Pinned to the radius rod is a 
block which slides in the link when the radius rod is raised or 
lowered by the engineer. When the block is above the link fulcrum 
the engine runs in one direction, and when below the fulcrum the 
engine runs in the opposite direction. 

The eccentric crank is so set that, when the engine is on either 
dead center the link stands in its middle position and if the radius 
rod were attached directly to the valve stem, the valve would 
also be in its mid-position regardless of whether the block were 
up or down in the link. However, when the piston is at the end 
of its stroke the valve should be displaced from its mid-position 
by an amount equal to the lap plus the lead. In the Walschaert 
gear this is accomplished by the use of a lever, called the lap and 
lead lever, which is attached to both the valve stem and the 
radius rod and is also connected, through a suitable rod, to the 
crosshead. This lap and lead lever is so proportioned that if the 
point of its connection to the radius rod be kept a stationary 
fulcrum and the engine piston moved a distance equal to the 
stroke the valve will be moved a distance equal to twice the lap 
and lead. When the piston is at the end of its stroke the valve 
is displaced from its mid-position a distance equal to the lap plus 
the lead. Inasmuch as the position of the valve, when the piston 
is at the end of its stroke, is dependent on the lap and lead lever 
alone, it is evident that the lead given by the Walschaert gear is 
the same for all points of cut-off. 

There are two arrangements of the Walschaert gear, de- 
pending on whether it is to be used with outside admission or 
inside admission valves. With a valve having outside ad- 
mission the valve stem is connected to the lap and lead lever at a 
point above the latter's connection to the radius rod (see Fig. 135). 
If the block is in the lower half of the link when in forward gear, 
the eccentric crank leads the main pin. If the block is in the 
upper half of the link when in forward gear, the eccentric crank 
follows the main pin. 

With a valve having inside admission the valve stem is con- 
nected to the lap and lead lever at a point below the latter's con- 
nection to the radius rod (see Fig. 136). If the block is in the 
lower half of the link when in forward gear the eccentric crank 
follows the main pin. If the block is in the upper half of the link 
when in forward gear, the eccentric crank leads the main pin. 



REVERSING GEARS 



127 



The diagrams in Figs. 137 and 138 represent the various valve 
events throughout a complete revolution of the wheels. Com- 
parison between corresponding diagrams in the figures shows 




No.6-0pen!n)j of rwnf Steam Porf /offfe 
^Bxhaust (Point of Fe/ease - Back Stroke). 



Fig. 137. — Walschaert gear and outside admission valve. Various im- 
portant positions during revolution. 




No. S—Opening of Front Steam Port fott7e 
''Exhaust (Point of ffelease-Back Stroke). 



Fig. 138. — Walschaert gear and inside admission valve. Various impor- 
tant positions during revolution. 

clearly the difference in the arrangement of the Walschaert 
valve gear for outside and inside admission valves. 



128 



VALVE GEARS 




Fig. 139 shows a Mallet 
articulated compound loco- 
motive equipped with the 
Walschaert gear. An in- 
teresting feature of this 
engine is that the forward 
cylinders, which are the 
low pressure, have flat 
valves admitting steam 
outside, while the rear 
cylinders, the high pres- 
sure, have piston valves 
admitting steam inside. 

The general arrangement 
of the Walschaert valve 
gear depends largely on 
the general design of the 
locomotive. The proper 
lengths of the arms of the 
lap and lead lever for any 
desired lap and lead may be 
determined by the formula: 

VXL 



S = 



2C 



in which 

>S = length of stroke 
(see Fig. 140.) 

V = distance from 
valve stem to 
radius rod con- 
nection. 

L — distance from 
radius rod con- 
nection to bot- 
tom connection. 

C = lap plus lead. 

It has been seen that 
with the Walschaert gear 
the valve receives motion 
from two different sources, 



REVERSING GEARS 



129 



the eccentric crank and the cross-head. For a rough analysis of 
the motion, it is convenient to determine some equivalent 
eccentric which acting alone would move the valve the same 
as it is moved by the combined action of the eccentric crank 
and cross-head. 

In Fig. 136, Og represents the link of the Walschaert gear in 
its mid-position, the trunnion being at 0. The dotted line Og' 
represents the link at some other time in the stroke. The 



Outside 
Admission 
Radius Bar 



Valve Stem 



Inside 

A d miss ion 



(Outside Admission) 



(Inside Admission) 




Note: 



Piston Stroke - 
S 



L ap and Lead Lever 
Connector to Stand 
Horizontal at Center 
of Stroke. 



Fig. 140. — Lao and lead lever. 



points g and g' indicate the positions of the block, and K and 
K' the end of the eccentric rod. The block attached to the 
radius rod is displaced the distance & and this displacement, 
slightly affected by angularity, is transmitted to the pin F 
connecting the radius rod with the lap and lead lever. 

The movement of the link to the dotted position is caused 
by a crank rotation through some angle Q which causes the 
eccentric center to move horizontally through the distance 
r sin Q and, neglecting angularity, the horizontal movement 

of F is r sin Q yd^, where Og and OK are straight-line 

measurements. Imagining the cross-head to be stationary, 



130 



VALVE GEARS 



the movement received by the valve from the eccentric is 

. Og (L-V) 
r sm e ^ — ^ 

In Fig. 141 draw any crank position and lay off OEi as shown, 

equal in length to r- %^^ --■'" -- -'-■-" ^^ ^^ " ^' 



Then Os = r sin 6 



OK L 

and OEi is the simple eccentric which is equivalent to the mech- 
anism operated by the eccentric crank when the radius rod is in the 
position shown in Fig. 136. If the block is above the trXmnion 
of the link, OEi should be laid off in the opposite direction in 
Fig. 141. 

Besides the motion received from the eccentric crank, through 
the link, the valve receives motion from the cross-head through 




Fig. 141. — Development of equivalent eccentric for Walschaert gear. 

the lap and lead lever. When the crank has moved through 
the angle d the point m (Fig. 136) is displaced from mid-position 

V 

a distance R cos Q, which is modified to R cos ^ y at the valve. 

This movement could be obtained from an eccentric OE2, (Fig. 

V 
141) set in line with the crank, where OE^. = R y) then 

V 
Ot = R cos Q Y ~ horizontal displacement of the valve due to the 

movement of the lap and lead lever. If an outside admission 
valve is used Oi/2 should be drawn on the other side of the shaft 
from the crank. 

It will be seen that when the cross-head is at the middle of 
its stroke, which will be when d is 90° if angularity is neglected, 



REVERSING GEARS 131 

the valve receives its motion from the link mechanism, repre- 
sented by the equivalent eccentric OEi, and Ot is zero. Also, 
when the cross-head is at one end of its stroke the distance Ot 
will be a maximum and Os zero, meaning that the valve receives 
its motion from the cross-head. By drawing E^E equal and 
parallel to OE2, sh = Ot, and Oh = Os -{- Ot. Therefore OE 
represents the single equivalent eccentric which would produce 
the same valve displacement as OEi and OE2 combined. 

It must be clearly understood that the use of an equivalent 
eccentric gives only the approximate valve motion and neglects 
any distortions due to angularity of the rods. In design work 
or careful analysis, it is necessary to lay out the gear very ac- 
curately on the drawing board and follow the various parts 
through a complete revolution to determine the exact movement 
of the valve. This should be done for at least two cut-offs, the 
latest one and the one at which the engine will be operated most 
of the time. 

In laying out the gear the following proportions represent 
good practice: 

Radius rod length at least 8 times the travel of the link block, 
10 or 12 is better. 

Eccentric rod length at least 3 1/2 times the eccentric throw; 
should be as long as circumstances will permit with practically 
equal lengths of radius rod and eccentric rod. 

Angle of swing of link should not be more than 45°. 

Angle of oscillation of lap and lead lever, 45° to 50° is good; it 
should always be less than 60°. 

The Walschaert valve gear can be designed to give a variable 
lead. This practice has been followed recently in a number of 
cases. With a variable lead the longest possible cut-off in 
starting can be obtained, combined with the proper amount of 
lead at the ordinary running cut-off. In the case of passenger 
locomotives particularly, a steam distribution like this is often 
most desirable. The favorable results for starting are obtained, 
however, at the expense of the distortion of the valve events 
in the back motion. For this reason, the Walschaert valve gear 
with variable lead is suitable only for passanger and fast freight 
locomotives, and not for slow freight and switching locomotives. 

With a variable lead so arranged that the lead increases as 
the reverse lever is hooked up, the eccentric crank lags behind 
the correct position for a constant lead; in other words, it is 



132 



VALVE GEARS 



so set that the link is not in its central position when the crank 
pin is on center. 

The Joy Valve Gear. — Fig. 142 shows a reversing gear, credited 
to David Joy, in which the use of an eccentric has been avoided 
altogether. The whole movement is derived from a point, a, 
near the middle of the connecting rod. Point d is fixed on the 
frame of the engine and k moves in an arc with d as a center, 
while points a and b trace oval curves as indicated in the figure, 
the major axis of the larger oval being equal to the stroke of 
the piston. As the point h traces its curve, point e slides up and 

i 

Waive Sfem_^^^ 





1 


\\ CoTi 


CejiferUrie_of_Cy]]nder /g~"^ 


■^:^^^^ 


WrisfPi^'^^ 


"^^rWf 


4^-^ - 


::z=£i 


r 


M^ 

Fixed Point 




on Engine Frame 




Fig. 142.- 


—Joy valv 


e gear. 



down in the curved guide or link. The point / of the rod /e& 
traces an approximate ellipse, and the travel of the valve is equal 
to the distance between two lines drawn perpendicular to the 
line of motion of the valve and tangent to the ellipse on either 
side. 

A change in the inclination of the link cause a change in 
the shape of the ellipse at / and a change in the valve travel and 
steam distribution. If the link is turned to the other side of 
the vertical axis, the engine will be reversed. 

Sometimes instead of the link a radius rod, Ae, is used, with 
h a movable point which can be held fixed in any desired position 
on the path }ih' . The same effect as shifting the link is then 
obtained by shifting the point A thus changing the center of the 
arc in which e moves. 

The equivalent eccentric can be found for any position of the 
link. The method is illustrated in Fig. 143, in which OEx is 



REVERSING GEARS 



133 



fe hk 



laid off along the crank line equal in length to , • , • OC, and 

60 (IK 

I -fe \ Wa 
E1E2 is perpendicular to OEi and equal to ( 1 -\ — 7 j -j^^OC-tan a, 

in which a is the angle of inclination of the link. The equivalent 
eccentric for the position of the link corresponding to the angle 
a is OE2. 

The validity of this construction can be proven by means of 
a skeleton diagram of the mechanism as shown in Fig. 144. 
The curved link or guide for the point e has been modified into 




Fig. 143. — Equivalent eccentric for Joy valve gear. 



a straight line and represented by AA. It is further assumed 
that the rod fg, connected to the valve stem, has no angular 
vibration, i.e., that the point g has the same movement as /. 
OF is laid off equal to aC and a line is drawn through F perpen- 
dicular to OW; this line will pass through the fulcrum of the 
link. If the pin at C were removed and the end of the connecting 
rod brought to 0, a would coincide with F, and the points /, e, a, 
h, k would fall on the vertical dotted line through F. 

Except for the effect of angularity, a receives the same hori- 
zontal movement as W. The point k travels on an arc and the 
combined movements of the points k and a counteract the dis- 
tortions due to angularity, so that the horizontal movement of 
the point h is almost perfectly harmonic. This being the case 
we may neglect the angularity of WC and assume that k travels 



134 



VALVE GEARS 



in a straight line; then the horizontal displacement of the point 
h, from the line through F, is given by the expression 

hi = -1 OC-cos 6 

The vertical displacement of a is equal to the vertical move- 

Wa 
ment of C multiplied by the ratio fr^rf- The distortions due 

to the angularity of the rod feb pivoted at e practically counter- 
act those of the rod ka pivoted at k, because they are opposite 




Fig. 144. — Skeleton diagram of Joy valve gear. 



in direction. The vertical displacement of the point e from the 
fulcrum of the link can then be written : 

V = ifjTTy OC sm Q 
WC 

On account of the path over which e travels being inclined 
from the vertical, the movement of e along that path causes 
e to be displaced horizontally as well as vertically. This hori- 
zontal movement is equal to v tan a, or 

Wa 



h^ 



WC 



OC sin 6 tan a 



REVERSING GEARS 135 

and must be acklecl to /)i to ji'ive the total horizontal movement 
of the point b; 

hi + h-i = -, DC cos d + „^ L/C sm d tan a 

The horizontal movement of the point / will be 

A- = /„ + (/„ + /,.)■;■;= {i+^,„+f^i„ 

/-. . fe\ Wa _^ . „ , , /^ bk ^^ 

= 1 + I, TT^^-OC-sm ^-tan a + -.■ ^ OC-cos 6 
\ ehl \\ C eb ak 

In Fig. 143, Om = OE^ cos d 

— ^ ■ T ■ OC cos d 
eb cik 

and mn = E1E2 sin d 

/. , fe\ Wa ^^ , . „ 

= ( 1 i — r I ^rrr^ OC tan a sm 6 

Therefore On = X 

and this movement can be secured from an eccentric OE^ set 
as shown in the figure. Hence the equivalent eccentric for the 
gear is OE2. 

PROBLEM 15 

In Fig. 144 take OC = 12", Ca = 4' 0", aW = 3' 0", ab = 7", bk = 12", 
dk = 2' 0"; perpendicular distance of d from WO = 1' 7", horizontal dis- 
tance of d from 0=6' 0"; bf = 2' 0", ef = 3", fg = 3' 6"; radius of link 
2' 0", center of curvature of link 7" above WO and 5' 11" to the left of 0. 

Find (1) horizontal displacement of 6. 

(2) vertical displacement of b. 

(3) travel of valve and angle of advance of equivalent eccentric. 



CHAPTER XII 

CORLISS VALVE GEARS 

In the United States, England and France the Corliss is the 
predominating type of large reciprocating engine. The form of 
the valve gear limits the speed of operation, and 100 to 125 R.P.M. 
represents usual practice. 

The Corliss valve gear was invented by George H.' Corliss in 
1850. Corliss realized that a radical change in design was neces- 
sary to improve the economy of the steam engine and the valve 
gear which bears his name is very different from the older types. 

Fig. 145 shows the common form of Corliss engine with the 
various parts of the gear labeled. Four valves are used, one in 
each corner of the cylinder, with their axes perpendicular to the 
cylinder bore and to the plane of rotation of the flywheel. On 
account of the location of the valves, the ports leading from 
the valve seats to the cylinder can be made short and the engine 
clearance thus kept low; it usually ranges between 2% and 6%. 
The two upper valves regulate the live steam, and the two 
lower ones the exhaust. 

Instead of sliding back and forth with straight line motion these 
valves move in an angular oscillation about their axes. This can be 
considered as a modification of and not essentially different from 
the ordinary sliding motion. Common forms of Corliss valves 
are shown in Fig. 146; they are made single ported and multiple 
ported, and the advantages of multiple ports are the same as in 
the case of slide valves. The valves extend clear across the 
cylinder in holes bored from side to side. These holes are closed 
at the back by flat plates and at the front by castings called valve 
bonnets. At each end of the valve a short, completely cylindrical 
portion forms the bearing. The valve is rocked by a valve spindle 
which has a bearing and a stuffing box in the valve bonnet and 
terminates in a tongue which fits in a slot extending diametrically 
across the front end of the valve. Fig. 147 shows a valve bonnet 
and the manner of connecting the valve and the end of the spindle. 

On account of the great distance between the crank-shaft and 
the cylinder it is necessary to add a rocker arm, dividing the 

136 



CORLISS VALVE GEARS 



137 




o 



^ 



138 



VALVE GEARS 



eccentric rod into two^ the reach rod and the eccentric rod proper. 
(See Fig. 145). 
Motion received from the eccentric is transmitted through the 






Fig. 146. — Common forms of Corliss valves. 







Fig. 147. — Corliss valve bonnet. 

eccentric rod, rocker, and reach rod to the wrist plate which 
oscillates about a stud fastened to the engine cylinder. Each 
valve receives a rocking motion by means of a valve rod con- 
necting it to the wrist plate. 



CORLISS VALVE GEARS 



139 



The Wrist Plate. — In Fig. 148 is shown a line diagram of the 
eccentric, rocker arm and wrist plate. The eccentric rod is 
attached to the rocker arm and as the eccentric turns from / to /' 
the end of the rod moves from I to l' and the rocker arm swings 




Fig. 148. — Line diagram of eccentric, rocker arm and wrist plate. 



through the angle jS. The distance IV is equal to the diameter 
of the eccentric circle, //'. At the upper end of the rocker 
arm is attached the reach rod running to the wrist plate. The 
movement of the reach rod and the point h is the movement 
of I magnified by the ratio of the lever arms hW and IW. 
G, with various subscripts, represents the pin on the wrist 
plate to which the reach rod is attached (see also Fig. 145) ; H 
with various subscripts represents the pin to which the steam 



140 VALVE GEARS 

valve rod is attached, and X the pin to which the exhaust valve 
rod is attached. 

The angle G1OG2 equals the angle H1OH2, equals the angle 
X1OX2, each being the angle through which the wrist plate is 
oscillated by the reach rod. Vertical lines have been projected 
from Gi and G2 and the horizontal distance between them, which 
equals the horizontal movement of the upper pin in the rocker 
arm, has been taken as the diameter of the path of an equivalent 
eccentric whose movement is the same as the one on the crank 
shaft but magnified by the rocker arm. 

When the eccentric turns from its inner extreme position to its 
outer extreme position Gi moves to G2, Hi to H2, and Xi to 
X2, with approximately harmonic motion. This motion in 
being transmitted to the valves, is considerably modified by the 
obliquity of the rods so that the motion of the valves is far from 
harmonic. The valves move rapidly in opening the ports, but 
slowly during the idle period. 

When the wrist plate is in mid-position the point which has 
extreme positions Hi and H2 (Fig. 148) stands at Hm- If the 
eccentric is at a for admission, the wrist plate has moved through 
the angle HJJHa from mid-position. Similarly, when release 
occurs and the eccentric is at r, the wrist plate has swung to the 
other side of mid-position through the angle GmOGr, which is 
equal to XmOXr, and . the pin connecting the exhaust valve rod 
stands at Xr. As the eccentric continues its rotation Xr swings 
to Xi, when the eccentric is at/, and then back again so that when 
the eccentric is at the compression position, k, the pin is again at 
Xr but moving toward the left. 

The Valve Motion. — A skeleton drawing is shown in Fig. 149 
in which Oi represents the valve center and the wrist plate 
center. With OiBi, the length of the valve arm as a radius, a 
circle has been drawn about Oi and tangent to this a line passing 
through 0. Then OHi represents the wrist plate arm, HiBi the 
steam valve rod and BiOi the steam arm, in one extreme position. 
This is often called the stretched position. If the total swing 
of the wrist plate is through the angle H1OH2, the other extrem.e 
position of these parts is shown as OH2B2O1. As the motion of 
the wrist plate is approximately harmonic the two halves of the 
angle H1OH2 are traversed in equal intervals of time. By 
drawing OHm midway between Hi and H2 and with the length 
of the valve rod as radius locating the point B^, it will be seen 



CORLISS VALVE GEARS 



141 



that while the wrist plate moves through half its angle, or HiOHm, 
the valve only moves through BiOiB„„ or about a third of its 
angle of oscillation. During the second half of the wrist plate 
motion the valve moves faster, traversing the angle B„,0iB2 
in the same time that it traversed BiOiB^. The period of 




Fig. 149. — Diagram of Corliss gear, showing effect of wrist plate. 

faster movement is used for the working period and the period 
of slower motion for the idle period. This movement of the 
valves due to the wrist plate effect is one of the advantages of 
Corliss engines. It is during the idle period that the valve is 
most unbalanced due to full steam pressure on one side. By 
reducing the amount of movement during this period the fric- 



142 



VALVE GEARS 



tional work necessary to operate the valve is greatly reduced. 
The rapid opening and closing during the working period reduces 
wire drawing and gives sharp corners on the indicator cards. 

It is not always possible to locate the centers and Oi in 
such a way that the angle OiBiHi is 90° without having the angle 
O1B2H2 very large. The latter angle should not exceed 140° 
because then the force along the link H2B2 would produce a heavy 
direct stress on the arm O1B2 and very little turning moment. 
The same consideration limits the angle OiBiHx to a minimum 
of 40°. When the angle OiBiHi is made less than 90° the angle 
BiOiBm approaches BmO\B2 and the wrist plate effect is 
reduced. Therefore it is desirable to make the angle OiBiHi 
as near 90° as possible. 

Trip Gear. — The mechanism on the steam valve spindle for 
operating the valve constitutes what is called a trip gear. There 



#°' 




Fig. 150. — Reynolds trip gear. 

are a number of trip gears for Corliss engines and descriptions 
of the common ones will be found in Power, Sept. 6 and 13, 
1910. Probably the best known and most widely used design 
is due to Edwin Reynolds, at one time chief engineer of the 
E. P. Allis Co. Figs. 150 and 151 show the essential parts of the 
Reynolds gear. Referring particularly to Fig. 151, a bell crank 
lever free on the valve spindle is continuously and positively 



CORLISS VALVE GEARS 



143 



rocked by the rod from the wrist plate. Attached to the l)ell 
crank is a hook or latch which is pressed in toward the valve spindle 
by a spring. This latch engages a projecting block on the steam 
arm which is the only arm keyed to the valve spindle. Thus to 
oscillate the valve spindle, the motion from the wrist plate must 
take the round-about path through the bell crank, the latch and 
the steam arm. 

The governor reach rod is attached to the governor arm G, 
which is free on the valve spindle. As the bell crank is moved 
counterclockwise to the position shown in Fig. 151b, the left hand 
side of the latch comes in contact with a knock-off cam on the 




(a) Latch T?aising Valve, 



(b) Latch Released 



Fig. 151. — Reynolds trip gear. 



arm G, which causes the latch to disengage the block on the steam 
arm and disconnect the valve from the wrist plate mechanism. 
The steam arm is attached to a dash pot rod which continually 
exerts a downward pull and tends to close the valve. As soon as 
the steam arm is disengaged from the latch the dash pot pull pre- 
vails and the valve is closed. As long as the speed of the engine 
is stable the position of the knock-off cam is unchanged, but a 
movement of the governor to secure equilibrium under a different 
load causes the cam to be shifted slightly about the spindle, thus 
changing the time at which the latch is disengaged and con- 
sequently the time of cut-off. As the engine slows down and 
the governor balls fall, G is rotated counterclockwise thus 
delaying the cut-off. A safety cam is usually provided so that 
if the arm G is moved to an extreme position due to the governor 
balls falling very low, the latch will not engage with the block, 
no steam will be admitted to the cylinder and the engine will 
stop. The safety cam will come into play in case an accident 



144 



VALVE GEARS 



to the governor belt or gears causes the governor to stop. It 
is necessary to have some means of preventing the safety cam 
from acting when the engine is being started. 

Two general arrangements of the releasing gear are in common 
use, known as inside hooks and outside hooks. The former is 
shown in Fig. 145 and the latter in Figs. 150 and 151. With 
one arrangement the valve admits steam by turning in one direc- 
tion and in the other by turning in the opposite direction. 

It is evident from the description of the action of the trip 
mechanism given above that the latch can be disengaged only 

while it is moving upward, and 
that if it is not pushed out by 
the cam during that part of 
its motion it will not be disen- 
gaged at all, but will retain its 
hold on the steam arm and 
the whole mechanism will 
move as though positively 
connected. 

The bell crank, Fig. 151, 
carries the hook upward as 
the wrist plate moves through 
the complete angle of oscilla- 
tion, that is, from one ex- 
This motion of the wrist plate is 
as it turns from one dead 




Fig. 152. 



treme position to the other 
derived from the eccentric which 
center position, e', Fig. 152, through half its revolution to the 
other dead center, m, moves the wrist plate through its entire 
angle of oscillation and draws the valve open by raising the steam 
arm. When the crank is on center, OD, the eccentric is at Oei, 
90° -f 5 ahead of the crank, and the crank can then rotate only 
through the angle 90° — 5 to the position OM before the eccen- 
tric reaches the position Om and changes the direction of motion 
of the wrist plate, carrying the latch downward and away from 
the knock-off cam. If the trip mechanism is to determine the 
time of cut-off it must act while the crank is moving through the 
angle 90° — 8. If the trip mechanism does not act, the cut-off will 
occur when the valve closes the port under the positive move- 
ment of the wrist plate, at crank position OCl. The range of 
cut-off between OM, the latest controlled by the governor, and 
OCl, the cut-off without the trip action, is entirely lost. 



CORLISS VALVE GEARS 



145 



The exhaust valves of Corliss engines are always positively 
driven either by the wrist plate which operates the inlet valves, by 




a wrist plate independent of the inlet valves, or by levers or 
rods which secure the same effect as the wrist plate. See Fig. 153. 

10 



146 



VALVE GEARS 



To cause release and compression to occur at the proper times 
requires a positive angle of advance of about 15° or 20° for the 
eccentric operating the exhaust valves. If only one eccentric is 
used, the inlet valves must be operated from the same wrist plate 
as the exhaust valves and on account of the angle of advance 
of 15° or 20° the usefulness of the governor is confined to early cut- 
offs as already explained in connection with Fig. 152. To in- 
crease the range of cut-off controllable by the governor and also 
to make the exhaust valves entirely independent of the inlet 




Fig. 154. — Near view of valve gear, double eccentric Corliss engine. 



valves, two eccentrics are often used. With only one eccentric 
the latest cut-off which the governor can control is about 3/8 
stroke, but with two eccentrics this range can be increased to 
about 3/4 stroke. 

If a separate eccentric is used to operate the inlet valves it 
can be given a negative angle of advance. This increases the time 
between the admission position and the extreme position of the 
wrist plate, making the trip operative over a wider range. When 
two eccentrics are used and the one for the admission valves is 



CORLISS VALVE GEARS 147 

given a negative angle of advance, the trip must always act or 
the engine will take steam during more than the entire stroke. 

Figs. 153 and 154 show the valve gear of the Allis-Chalmers 
Corliss engine with independent admission and exhaust valves. 

Design of Corliss Valves. — General Considerations. An im- 
portant consideration in designing Corliss valves is to make 
them sufficiently stiff and rigid. The width of the valve 
is always small in proportion to its length but strength can be 
secured by properly shaping the body of the valve as illustrated 
in Fig. 146. Sometimes it is advisable to add stiffening ribs. 
Regardless of the shape chosen, care must be taken to see that 
the passageway for the steam is not obstructed. 

In locating the valves and shaping the metal of the cylinder 
casting around them, provision should be made for removing 
the condensed steam, and no pockets or hollows should be left 
which will hold water. 

It is convenient to have the inlet and exhaust valves of the 
same diameter. For preliminary purposes this can be taken as 
1/3 to 1/5 the cylinder diameter. 

The exhaust valves are made to operate over the ports lead- 
ing to the exhaust chamber and not those leading to the cylinder. 
This allows the steam pressure to hold the valve firmly against 
its seat. Since this construction tends to increase the clearance 
volume it is necessary to shape the valve so that only the re- 
quired passage area is left open. 

In designing Corliss valves the port openings are computed in 
the same manner as for slide valves. With single eccentric gears 
the admission ports can be figured on a nominal steam speed of 
8000 ft. per minute. For double eccentric gears the nominal 
steam speed should be taken as 7000 ft. per minute. 

Layout. — The method of laying out a Corliss gear can best be 
described by an example. Let it be required to lay out the 
valve gear for an 18" X 36" 100 R.P.M. horizontal Corliss 
engine, to have one wrist plate and to give release at 98% of the 
stroke and compression at 96%, the valves to be single ported 
and the gear to have the hooks inside. 

Computation for admission port : 

2 X 12 X 100 X "^^^^ = 8000 X area of port 
Port area = 19.1 sq. in. 



148 VALVE GEARS 

The length of the port in a Corliss engine is usually equal 

to the diameter of the cylinder, which in this case is 18". The 

19.1 
width of the admission port is therefore— r^ = 1.06". 

lo 

Similarly the exhaust port must be 

1.06 x|^ = 1.4" wide. 

The width of the piston for such an engine would be about 
5" and the clearance distance at each end of the cylinder 1/2", 
making the length between cylinder heads 

36" + 5" + 1" = 42". 

A longitudinal section of the cylinder would show the bore as a 
rectangle 18" X 42". 

The diameter of the valves should be proportional to the 
diameter of the cylinder and should be from 1/3 to 1/5 the 
cylinder diameter. This gives extreme values of 

18 18 

-— = 6" to -^ = 3.6", from which a conservative diameter of 5" 
6 5 , 

can be chosen for preliminary purposes. All the valves should 

be of the same diameter. 

Allowing 1" for the thickness of the metal of the valve seat 

and the counterbore of the cylinder, the vertical distance between 

the centers of the valve should be as shown in Fig. 155: 

18"+ 5" + 1" + 1" = 25". 

The centers of the valves may be located in line with the inside 
surfaces of the cylinder heads but this is often modified to ob- 
tain more advantageous forms of ports. The important con- 
sideration is to keep the clearance volume down to a minimum. 
The wrist plate is located at the intersection of the diagonals 
passing through the centers of the valves. 

The length of the valve arms may be taken equal to the 
diameter of the valves, and the wrist plate arm, or the distance 
from the fulcrum of the plate to the pins, as 1 to 1| times the 
length of the valve arms. Taking as an average Ij, the length 
of the wrist plate arm is Ij X 5" = 6j". These proportions 
are for preliminary purposes only and it remains to be seen by 
a careful layout on the drawing board whether they will give 
satisfactory results. 

Starting with the location of the valves and wrist plate as 



CORLISS VALVE GEARS 



149 



shown in Fig. 155, scribe arcs around and Oi to represent the 
ends of the wrist plate arm and steam valve arm respectively. 
Also draw a circle to represent the valve seat. 

Ordinarily the total angle of oscillation of the wrist plate is 



fjmmmmm 




Fig. 155. — Corliss layout. 

from 60° to 90'^, with 75° as a very good average. This angle 
was first taken as 75° and the mechanism drawn in the ex- 
treme position OHiBiOi with OHiBi a straight line tangent 
to the arc B1B2 representing the path traversed by the valve 



150 " VALVE GEARS 

arm. In other words the angle HiBiOi was made equal to 
90° and the angle OHiBi equal to 180° for a preliminary 
trial, with 75° for the angle of swing of the wrist plate. The 
wrist plate was then imagined swung through the assumed angle 
of oscillation (75°) and the other extreme position of the mechan- 
ism was drawn. The angle between the valve arm and the 
valve rod was then investigated to see if it came within the 140° 
limit. Often this angle comes too large, if in the other extreme 
position it is about 90°, and it is then necessary to assume a 
longer valve rod. This will cause the angle HiBiOi to be less 
than 90° but it should always be kept greater than 40°. In this 
particular problem, with a 75° angle of oscillation for the wrist 
plate, if one angle is made 90° the other works out 153.5° with 
a valve rod 17 11/16" long and the rod must be increased in 
length to 18 1/4" before the larger angle falls within 140°. 
This causes the smaller angle to be reduced to 83°. Sometimes 
it is necessary to change the length of the valve arm before 
satisfactory conditions are secured. The designer should not be 
satisfied until he has an arrangement which will make the angle 
HiBiOi as near 90° as possible without having H^BiOi exceed 
140°, because then he will have the largest wrist plate effect. 
The mid-position of the mechanism should be drawn and 
the ratio of the valve arm throws examined; the first layout 
showed one angle to be 19° and the other 48°, a ratio of 48/19 = 
2.52; average values are 2.5 to 3. If the angle of movement of 
the valve is too large the valve arm should be lengthened or the 
swing of the wrist plate reduced. 

Before the design is carried any farther it should be noted 
whether the valve can be expected to give the necessary port 
opening. A certain steam lap may be arbitrarily assumed and 
then, by swinging the valve to its extreme position, the probable 
port opening can be determined. If this is thought to be too 
small or too large it can be modified by changing the length of the 
valve arm, or the swing of the wrist plate, thus changing the angle 
of oscillation of the valve. The Allis-Chalmers Company recom- 
mends the following values for lap and lead: 



Cylinder diameter 


Steam lap 


Exhaust lap 


Lead 


8" to 14" 


3/16" 


1/16" 


1/32" 


14" to 22" 


1/4 " 


1/8 " 


1/32" 


22" to 32" 


5/16" 


3/16" 


3/64" 


32" to 36" 


3/8 " 


1/4 " 


1/16" 



CORLISS VALVE GEARS 



151 



In the layout under consideration the 48° of rotation from the 
mid-position to the extreme position caused a movement of 2.1'" 
at the valve face and subtracting 1/4" for the assumed steam lap 
gave a net opening of about 1.85" which would be too large, since 
the computed amount was only 1.06." Three possible changes 
to reduce this port opening were considered, viz : 

1. Lengthening the valve arm. 

2. Reducing the swing of the wrist plate. 

3. Reducing the diameter of the valve. 

It was decided to reduce the swing of the wrist plate to 65° and 
a new layout of the mechanism was made. In the new layout, 




Fig. 156. 



Fig. 157. 



with the angle at Bi equal to 90° the angle at B2 came 130° and 
the ratio ^T^ p" = -r-rr = 2.6. The total swing of the valve 

BlUlBm 14:. O 

measured 52.1 degrees. With this new mechanism the steam 
port opening promised to be about 1^" and the exhaust port 
opening about 1^". These were considered satisfactory. 

From the given data it is possible to construct a diagram giving 
the positions of the crank for the various events, as shown in 
Fig. 156. Release and compression are given, thus determining 
the angle of advance as 20°; admission may be chosen at 3° 
before dead center. Since the engine is equipped with a trip 
gear the cut-off is ordinarily entirely independent of the wrist 
plate movement. If, however, the engine should be so heavily 
loaded that the trip would fail to act, the valve would be closed 
by the wrist plate when the crank reached the position OC (Fig. 
156). By rotating Fig. 156 clockwise through the angle 90° + 5 



152 VALVE GEARS 

the various crank positions become corresponding eccentric po- 
sitions and Fig. 157 results. 

Fig. 157, magnified or reduced as explained in connection with 
Fig. 148, is then drawn above the wrist plate in Fig. 155 so that 
the vertical axes coincide, and the diameter of the eccentric circle 
equals the horizontal movement of the point G on the wrist plate. 

At the time of head end admission the eccentric would be at a 
(Fig. 155), the reach rod pin at Ga, the steam valve rod at HaBa, 
the valve arm at BaOi, and the left hand edge of the valve at 2 
just ready to admit steam as the valve turns counterclockwise. 
The exhaust valve rod would at that instant be at XaDa with the 
valve closed and rotating counterclockwise. 

As the eccentric continues to revolve Ga moves to G2, Ha to H2, 
Baio B2 and the extreme edges of the steam valve from 2 and 9 
to 5 and 10. At the same time the point Xa moves to X%, Da to 
D2 and the edges of the exhaust valve from 3 and 11 to 4 and 12. 

On the return swing of the wrist plate (swinging counter- 
clockwise) when the mechanism is in the mid-position, the edge 
2 of the steam valve will have slipped past the port by an amount 
equal to the arc between points 4 and 6. This distance is there- 
fore the steam lap. Similarly the distance between the two points 
6 and 7 on the exhaust valve is equal to the exhaust lap. 

The two extreme positions of the steam valve bring the edges 
in one case to 1 and 7 and in the other to 5 and 10. Both points 
1 and 10 should be beyond the metal of the cylinder casting to 
avoid wearing shoulders; 7 must be sure to seal the port, with 
some margin; 5 should provide sufficient opening. 

The exhaust valve is shaped so as to give a uniform passage for 
the exhaust without any unnecessary space to increase the 
clearance volume. When wide open with X at Xi, the edge 11 
is at 10 and the edge 3 at 1. The distance between the points 2 
and 1 is the maximum exhaust opening and point 2 is the point of 
opening and closing when release and compression occur. 

Only one end of the cylinder has been discussed, for simplicity 
and because the solution for both ends would necessitate con- 
siderable reduction in the size of the drawing; furthermore, the 
solution for the other end is so similar that additional explanation 
seems unnecessary. 

The Dashpot. — When the trip mechanism of a Corliss engine 
disconnects the hook from the valve the dashpot closes the 
valve. The dashpot has two functions to perform: 



CORLISS VALVE GEARS 



153 



1. It must exert a downward pull on the valve. ^ 

2. It must be self cushioning. 

Several different forms of dashpots are in common use; 
they are all similar in action. As the wrist plate mechanism 
draws the valve open, the dashpot rod (Figs. 145 and 150), with 
the plunger at its lower end, is raised. A partial vacuum is thus 
formed under the plunger and the atmosphere then exerts a 




H. 



Cylinder- f\ 



Reg. Valve 







Base Plate 



m. 



Fig. 158. — Typical forms of dashpots. 

downward pressure which is ready to force the valve closed as 
soon as it is released by the trip. To reduce shock and pre- 
vent the dashpot from being noisy air is used as a cushion to 
bring the parts quietly to rest. 

Typical forms of dashpots are shown in Fig. 158. The first 
illustrates the single chamber and the other two the double 
chamber type. Referring to III, which is a section of the 



154 



VALVE GEARS 




CORLISS VALVE GEARS 



155 



Vilter dashpot, when the plunger is raised a partial vacuum 
is formed beneath it; the completeness of this vacuum and there- 
fore the downward force exerted can be adjusted by the regulating 
valve. This valve allows air to enter below the plunger as 
desired. When the plunger descends, the air which has been 
allowed to enter through the regulating valve, is forced out through 
the air valve which is of the ball type. Although this air valve 
is also adjustable, very little cushioning is expected from the 
inner cylinder. Its duty is to provide the downward pull. 
The outer cylinder is to give the cushioning action. By adjusting 
the cushion valve the rate of flow of the air through the passage 
C can be regulated to give the desired cushion. 

FOUR-VALVE ENGINES 

Engines equipped with four Corliss valves, 'positively con- 
nected, are called four-valve engines. The mechanism is designed 
so as to give to the valve practically the same movement as in 




( Greatest- Movement'[ 
\ of Eccentric Rod \ 



Fig. 160. — Movement curve of Armstrong gear. 



the Corliss engine. At the proper time the valve is opened 
quickly and then closed quickly and held almost stationary 
during the idle period. In the Corliss engine the steam valves are 
momentarily disconnected from the eccentric mechanism when 
the trip acts, but the valves of a four-valve engine are positively 
connected and under the influence of the eccentric at all times. 
The Armstrong non-detaching valve gear used on the Ball 
four-valve engine is a good example of this type. Fig. 159 shows 
the general arrangement of the mechanism. A diagram in which 
the movement of the eccentric rod is represented horizontally. 



156 



VALVE GEARS 



and the resultant movement of the valve, vertically, is shown 
in Fig. 160. The valve and the valve gear case are shown in 
Fig. 161, and Fig. 162 shows the case with the cover re- 




FiG. 161. — Armstrong non-detaching gear. Valve and valve gear case. 



\ 




D 




\ 


\ j^^^' * 




, ' ''I 


A 




I^^H' 




m 


^■♦MP-T^ia 


IRp 


■--.f 1 


9 


^■»tm!^? 


^^" 


'S 


G-r" 


I^H^S 


^. « 


W 


1 




B 


^ 


\ 


\ 



Fig. 162. — Armstrong non-detaching gear. Cover of case removed. 

moved. The gear is shown in elementary form and in several 
consecutive positions in Fig. 163. By following the motion of 
the various parts, it may be seen that the link CD, connecting with 



CORLISS VALVE GEARS 



157 




a 
o 
o 



158 VALVE GEARS 

the valve crank YD, simply swings around the pin D without 
imparting any motion to YD for one-half of the total movement 
of the eccentric rod which drives it. At usual loads the valves 
have their highest speeds at the instants of opening and closing. 
At each valve all the parts of the gear, except the main driving 
arm, are enclosed in a tight casing filled with oil. 



CHAPTER XIII 



POPPET VALVE GEARS 



Poppet or Lift Valves. — The poppet or lift valve has long been 
predominant in German stationary practice in the field where the 
Corliss valve has been used in the United States. The poppet 
valve is fundamentally different from the slide and oscillating 
valves. Instead of sliding or rocking over a seat to uncover a 
port, the poppet valve lifts from the seat 
with a movement perpendicular to the port. 
The slide valve is in constant motion but 
the poppet valve, after seating, remains at 
rest until the time comes for it to open 
again. The main advantage of the poppet 
valve is due to the fact that it has no move- 
ment on the seat, thus requiring no lubri- 
cation and being particularly adapted to 
the use of superheated steam. The four 
valves are independent of each other and 
therefore very precise adjustments in the 
diagram points can be made. 

Poppet valve engines are limited in speed 
on account of the inertia forces of the 
closing valves. Usually they are operated 
at about the same speeds as Corliss engines 
although 150 or even 200 R.P.M. are at- 
tainable in some cases. 

Typical forms of poppet valves are shown 
in Figs. 164, 165 and 166, the last being 
quadruple seated and the others double 
seated. All are designed so as to be nearly 
balanced. In order to place the valve in 
position the seatings must have different 
diameters. The inside diameter of the larger seating should be 
the same as the outside diameter of the smaller seating so that 
the unbalanced area exposed to steam pressure will not be larger 
than necessaiy. When the valve is closed the total area subjected 

159 




Fig. 164.— Double 
seated poppet valve 
and bonnet. 



160 



VALVE GEARS 



to an unbalanced pressure is only an annulus equal to or slightly 
exceeding the combined width of the surfaces in contact. A 
small unbalanced pressure is desirable because it tends to keep 
the valve steam tight and prevent accidental opening. 

Four poppet valves are necessary on a double acting engine, 
two for admission and two for exhaust. In the study of slide 




Fig. 165. — Double seated poppet valve showing position in cylinder. 



valves it was seen that only one valve was necessary on a double 
acting engine and that it could be made to connect each cylinder 
port alternately with the steam and exhaust ducts; in other words, 
one chamber was connected alternately with two other chambers. 
A poppet valve can connect or disconnect only two chambers. 
The valve stem is always vertical. 

A casting called the valve basket (see Fig. 165) contains the seats 
for the valve and four such castings are inserted in the cylinder 
casting. The valve and basket should be of the same material 
and cast from the same heat so that both will deform equally 
under the influence of heat and in that way prevent warping. 



POPPET VALVE GEARS 



161 



Usually they are made from the best grades of cast iron. To 
secure perfect fits the valve is ground on the seats. The most 
particular manufacturers do this work with the valve and basket 
exposed to steam at the pressure which will be used in the engine. 
The valve should be light, and guided so tliat it will seat cor- 
rectly. Sometimes fins are used which slide on the valve basket 




Section! A - B . 

VafveBorskef 




Fig. 166. — Quadruple seated poppet valve. 



and keep the valve central. Usually the valve stem extends 

beyond the valve and by sliding in a small cylindrical cavity 

in the disk of the basket keeps the valve concentric with the 

seats. It is not sujfficient to depend only on the stuffing box 

and stem. 

Different practice is followed in regard to the angle of seating. 
11 



162 



VALVE GEARS 




Sometimes the seat is made flat 
and sometimes conical, tiie angle 
of the cone with the horizontal 
varying from 30° to 60°. The 
inclined seat does not cause so 
much change in the direction 
of flow of the steam; also the 
speed of the valve, perpendicular 
to the inclined seat, is less than 
the vertical speed, thus reducing 
the valve hammer. Some au- 
thorities advise making the cones 
of the two seats have a common 
apex, it being assumed that under 
the action of heat the valve is 
deformed in all directions equally. 
Then, even though the seats 
move a trifle, proportionality is 
preserved, and the fit remains 
tight. The horizontal width of 
the seating is small, usually 1/8" 
to 1/4". 

As in the other types of valves 
the size and proportions of a 
poppet valve are governed by the 
required passage areas, which in 
turn depend upon the allowable 
steam speeds. The same nominal 
steam speeds as were used for 
Corliss valves may be applied to 
poppet valves. All the steam 
flowing through the valve must 
pass through one circular opening 
whose diameter may be desig- 
nated as D. This circular open- 
ing is not clear but is obstructed 
by the valve stem, the valve hub, 
the body of the valve and any 
ribs or fins that may form a part 
of the valve. In a preliminary 
calculation it may be assumed 



POPPET VALVE GEARS 



163 



that 20% to 40% of the total area under the upper seat is ob- 
structed. The net unobstructed area would then be 

)2 



A = (0.6 to 0.8) ^ 



or 



D 



-4. 



4A 



(0.6 to 0.8)7r 
= (1.45 to 1.26) Va 
The lift if a double seated valve, to give this opening, would be 
determined as follows: 

0.5 A = h{irD - W) 
where W is the sum of the widths of the ribs of valve and basket 
which reduce the free circumference 

0.5 A 0.5A 



h = 



ttD-W 7r(1.45to 1.26) Va-TF 




Fig. 168. 



-Sectional elevation of admission valve, showing 
constructional details 



The general arrangement of a poppet valve engine is shown in 
Fig. 167, which is a longitudinal and transverse section of the 
Lentz Engine built by the Erie City Iron Works. The valve is 
double seated and is shown in detail in Figs. 164 and 168. There 
are no stuffing boxes for the valve spindles. The latter are ground 
to fit the long bushings within 0.001 " and grooves are turned in 



164 VALVE GEARS 

the spindles to prevent leakage. This form of construction is 
particularly advantageous with superheated steam. The valves 
are turned to such diameters that the lower ring will just pass 
through the upper opening. No dash pots are used. T he valve 
is moved by a cam acting on a roller. When the valve is seated 
the cam is not in contact with the roller, but the amount of clear- 
ance is too small to be shown in the drawing. The roller remains 
in contact with the cam until the valve is seated. 

Alongside of the engine runs a horizontal lay shaft which is 
caused to rotate by a pair of bevel gears between it and the crank- 
shaft. On the shaft are grouped eccentrics, one for each of the 
valves, and the rods of these eccentrics are coupled directly to the 
ends of the cam levers. The steam eccentrics are under the con- 
trol of an inertia governor on the lay shaft. This governor shifts 
the position of the eccentric center and thus regulates the point 
of cut-off. 

The clearance in engines equipped with poppet varies con- 
siderably with the form of the valve and the arrangement of the 
valves with reference to the cylinder. Usually the clearance 
will be from 4% to 10%. 

Poppet Valve Gears. — The valves usually derive their motions 
from a lay shaft running parallel to the center line of the cyl- 
inder (see Fig. 167) and driven by the crank shaft through a 
pair of mitre gears. On hoisting and other reversing engines 
equipped with link motion, however, the valves are operated 
directly from the crank shaft. In the case of vertical engines 
the motion is sometimes taken directly from the crank shaft but 
usually an intermediate shaft must be added. 

Between the lay shaft and the valve there may be either a 
positive or a trip motion. With the former the valves are lifted 
and returned to their seatings by the mechanism or in accordance 
with the motion of the mechanism. In the latter case the valve 
is not closed by the mechanism but by some special closing force, 
such as a spring, which pushes the valve back to its seat as fast 
as the mechanism connected to the valve will allow it to reseat. 

When trip motion is used the valve is opened positively until 
at some instant, controlled by the position of the governor, the 
valve stem is disconnected from the mechanism and the valve is 
automatically closed by a spring which is always ready to act. 
An air or liquid dashpot is provided to reduce or eliminate shock 
when the valve seats. A very good example of a liquid dashpot 



POPPET VALVE GEARS 



165 



is shown in Fig. 169. The position of the finger on the governor 
shaft determines the time when the hook will be disengaged and 
the lower spring allowed to close the valve. ' As the valve 
descends, the oil in the dashpot above is displaced from the 
lower to the upper side of the plunger through holes in the 




Fig. 169. — Double seated poppet valve and bonnet, showing dashpot. 



sides. The area of the openings for the passage of the oil 
diminishes as the valve nears its seat and this dampens the 
valve and causes it to seat quietly. 

With the common arrangement of a lay shaft from which the 
valves are operated, four eccentrics are used, one for each valve. 
The eccentrics for the inlet and exhaust valves on one end of the 



166 



VALVE GEARS 



cylinder are attached to the lay shaft diametrically opposite those 
for the other end of the cylinder. Any eccentric can be adjusted 
without affecting any other; thus one valve motion is independent 
of all the others. 

The mechanism must be constructed so that after the valve 
is seated the motion of the gear will not affect the valve until the 

time comes for it to open 
again. When a trip is used 
this is obtained quite natur- 
ally but with a positive gear 
the desired result can only be 
obtained by using some form 
of rolling lever, rotating cam 
or oscillating cam. 





Fig. 



170. — Poppet valve bonnet 
showing rolling lever. 



Fig. 171. — Cam and roller. 



A rolling lever is shown in Fig. 170. The fulcrum of this lever 
is the changing point of contact between it and the curved surface 
over which it rolls, when the valve is open, but when the valve is 
closed the lever is lifted from the curved surface and the fulcrum 
becomes the pin at the valve stem. Any movement of the lever 
after it leaves the curved surface does not affect the valve. The 
lever and its support are designed so that the motion of the valve 
is slow when near its seat but rapid in giving the full opening. 
This is effected by the varying lever arms as the fulcrum moves 
away from the valve in opening and approaches aga,in in closing. 
The lever does not close the valve but the valve is returned to its 
seat by a spring as the lever allows it. When the valve is seated 
the fulcrum of the lever is at the valve stem end and the valve is 



POPPET VALVE GEARS 167 

held closed by the spring until the lever and the plate again come 
into contact. 

It is important to design the rolling lever so that it will roll 
and not slide over the support. 

If rotating cams are used on the lay shaft instead of eccentrics, 
the cams can be so shaped that the valve will be lifted and allowed 
to return to its seat at the proper times. In such cases the rods 
to the valves terminate in rollers which are in contact with the 
cams as shown in Fig. 171. 

An oscillating cam, or cam lever as it is sometimes called, is shown 
in Fig. 168. This cam is rocked by the eccentric rod and the 
shape of the curved surface causes the valve to open and close at 
the proper times. The cam also determines the amount of lift and 
it must be shaped so that the valve will remain seated during the 
idle period. By properly shaping the curves of the cam, shocks 
at the beginning and end of the valve movement can be avoided. 

Spiral steel springs are generally used to supply the necessary 
closing force whether the valve is operated positively or by trip 
mechanism. If the valve lowers for closure, the spring maybe 
computed as follows: 

Let F = mean force of the spring 

Q = necessary force to close the valve 
W = weight of the valve and stem, in pounds 
t = time of closure in seconds 
h = valve lift in feet 
a = acceleration of valve 
g = acceleration due to gravity 

Then h = "^ 

2h 
a = —- 

t^ 

Q = a 

9 

aW 

9 
g 9 16.2 ^- 

The allowable closing time may correspond to about 12° of crank 
rotation. 



168 VALVE GEARS 

In making a layout of a poppet valve gear the first step is to 
assume the general proportions of the entire mechanism, i.e., 
the location of the various pins and fulcrums, the lengths of the 
rods and the throw of the eccentric. These dimensions should be 
so chosen that the valve lift will be sufficient under all probable 
conditions of operation. It is a good plan to draw the valve lift 
curves which the assumed mechanism will give and then compare 
these curves with the theoretical valve lift eUipse which is de- 
termined by plotting the piston positions against the computed 
necessary port openings based on the allowable steam speeds. 
This comparison will usually suggest desirable changes in the 
assumed mechanism and modifications should be made until 
satisfactory results are obtained. Finally a careful layout should 
be made in which the successive positions of each pin or other 
important part of the mechanism is shown for an entire revolution. 



INDEX 



Admission, 7, 8, 11, 14 

lines, 11 
Allen valve, 65 
Angle of advance, 13 
Angular movement of connecting 

rod, 53 
Angularity of connecting rod, 53 

of eccentric rod, 12 
Areas of ports and passages, 40 
Armstrong non-detaching gear, 155 

Back pressure, 7 
Balance ring, 70 

valves, 71 
Ball engine valve, 66 

and Wood telescopic valve, 67 
Bilgram diagram, 17, 21 
Blake-Knowles valve gear, 117 
Bridge, 52 
Buckeye engine valve, 108 

governor, 75 
Bushings, 59, 60 

Cameron pump valve, 114 
Cam levers, 167 

oscillating, 167 

rotating, 167 
Centrifugal governors, 73 
Characteristics of Zeuner diagram, 

20 
Compression, 7, 8, 14 
Connecting rod, 2 
Corliss valve gears, 136 

valves, design of, 147 
Crank and piston, relative positions 
of, 1 

circle, 1 

pin, 2 

positions corresponding to cer- 
tain eccentric positions, 15 
Cross-head, 2 
Curves, displacement, 56 
Cut-off, 6, 8, 14 



D shde valve, 2, 6, 8 

Dashpot, 152 

Design of Corliss valves, 147 

of slide valves, 84 
Direct valves, 63 
Displacement curves, 56 
Double eccentrics for Corliss engines, 

146 
Double-ported flat valves, design of, 
91 

piston valve, 65 

Eccentric, 2, 4, 5, 14 

analogy between crank and, 4 

positions transferred to crank 
positions, 15 

rod, 2, 5 

angularity of, 12 

sheaves, 5 

straps, 5 
Exhaust lap, 8, 9, 10 
Expansion curve, 7 
Extreme position of valve, 8 

Flat valves, 58 

design of double-ported, 91 
Forms of slide valves, 58 
Four-ported valve, 67 
Four-valve engines, 155 
Fundamental idea of Bilgram dia- 
gram, 22 
of Zeuner diagram, 17 

Governors, Ball and Wood, 96 
Buckeye, 75 
centrifugal, 73 
inertia, 73 
Rites, 76 

Robb-Armstrong, 77 
shaft, 73 
Westinghouse, 74 

Indicator diagrams, 6, 11, 82 
Indirect valves, 63 



169 



170 



INDEX 



Inertia governors, 73 
Inside admission valves, 63 

Joy valve gear, 132 

Lap angle, 13 

exhaust, 8, 9, 10 

steam, 8, 9, 10 
Lead, 10, 11, 12, 13 

angle, 13 
Lentz engine valve gear, 163 
Lift valves, 159 

Location of ports in valve seat, 70 
Locomotive cylinder casting, 62 

piston valve, 60 

Maximum port opening, 19, 22, 48 
Mclntosh-Seymour valve gear. 111 
Meyer valve, 101 

Mid position of slide valve, 8, 9, 13 
Multiple ported valves, 63 

Negative lap, 10 

Obliquity of connecting rod, 54 
Outside admission valves, 63 
Overtravel, 56 

Piston, 2 

and crank, relative positions, 1 

rod, 2 

valves, 58, 59 

velocity, average, 41 
instantaneous, 40 
Poppet valves, 159 

valve gears, 164 
Port areas, 40 

opening, 10, 40 
Ports, location of, in valve seat, 70 
Pressure plate, 66, 69 
Pump valves, 114 

Relative positions of piston and 
crank, 1 
of valve, eccentric, and crank, 
14 
Release, 7, 8, 14 
Reversing gears, 120 
Riding cut-off valves, 100 



Rites inertia governor, 76 
Robb-Armstrong governor, 77 
Rockers, 71 
Rolling levers, 166 
Rotating cams, 167 

Shaft governors, 73 

investigation of action of, 96, 97 
Skinner engine valve, 70 
Slide valves, design of, 84 

forms of, 58 
Sliders, 2, 71 

Solution of typical problems, 25 
Steam chest, 2, 6 

lap, 8, 9, 10 

speeds, instantaneous, 41 
nominal, 42, 48 
Stephenson link motion, 120 
Stroke table, 55 

Telescopic valve, 67 
Trip gear, Reynolds, 142 
Typical problems solved by both 
diagrams, 25 

Use of valve diagrams, 25 
Useful characteristics of Zeuner dia- 
gram, 20 

Valve diagrams, 17 
use of, 25 

displacement curves, 56 

ellipse, 84 

functions of, 9 

gears, 1 

setting, 80 

stem, 2, 5 
Valves, forms of slide, 58 

with riding cut-off, 100 
Velocity of piston, 40 

Walschaert valve gear, 123 
Westinghouse governor, 74 
Wire drawing, 40 
Worthington duplex pump, 115 
Wrist plate, 1 39 

Zeuner diagram, 17 



